pblanc is right. Fiberglass, kevlar and other composite materials can be formed into complex shapes that are much more aerodynamic with compound curves. They are stiffer and stand up better to UV light.
More hydrodynamic too! 
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Iāve only seen plastic boats get bent on roof racks in the hot Florida sun.
Here is how I summarize the 3 main materials (but of course, lots of variations, so this is how I see for standard boats of each material):
Composite are light, stiff, and repairable, at the cost of being expensive and a little more fragile. They are made of fiberglass, carbon, or kevlar. They can be made in smaller numbers, so customization and more complex shapes are easier. $4+k new and maybe 50 pounds.
Thermoformed (current Eddylines) are almost as light and as stiff and somewhat repairable, but not as expensive as composite. They are made of some form of ABS plastic. $2.5k new for comparable boat and maybe 55 pounds.
Rotomolded plastic (the majority of boats out there) are heavy, cheap and nearly indestructible, but not as stiff (so not as efficient through the water), can get fuzzies hanging off as they wear (adding to the inefficiency through the water), and almost impossible to fix when you do destroy one. They are made of some form of polyethylene and are made in molds that to be cost effective, they need to spit out hundreds or thousands of the boats over the life of the mold. $1.5k new for a comparable boat and maybe 60 pounds.
So back to your question - why buy something other than plastic? I find the inefficiencies of rotomolded plastic going through water to be nominal, so doesnāt bother me. But the lack of repairability could be an issues. If you do wear down the plastic or punch a hole through it, you could try to plastic weld it, but in general the plastic weld will not be as strong as original so will always be a weak spot (and may not hold on at all - the older and more aged the plastic on the boat, the less likely a repair will hold). So most expeditions use composite boats, because if they bust a hole in a boat one day, they could use basic fiberglass repair materials to get it working again by the next day. With a plastic boat youād be screwed.
And as talked above, composite boats can be more efficient, so are used by racers and performance enthusiasts.
My 2000 Falcon has a rather has rather deep vee hull that transitions into rounded chines and I really expected a squirrely feel, but itās less tippy than I expected. Not rock solid like a flat bottomed boat, but it does want to stay upright if you relax versus feeling like it wants to roll. Iām 6ā and 165 lb with wimpy arms and chest, so my center of gravity might be low enough to help.
It is like good sex; until you experience it no one can really explain it. Composite boats have an edge you canāt find in plastic.
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Eddyline is not as stiff as most composite hulls.
True, and some of the multi-layer poly boats are nearly as stiff, though heavier. Technology has certainly improved and generalizations that were true in years past arenāt quite so accurate today.
The hull and paddling characteristics are much more important to me than the specific material.
Good summary Peter-CA
Delta kayaks are also ABS moulded and are a very nice looking kayak and the weight is comparable to fiberglass kayaks.
Seems likely for Eddylines built after they switched to ABS plastic. But my old Kevlar composite Eddyline is a stiff as my other composite boats.
Good point. My reply was specific to the Carbonlite Eddylines. I currently own an Eddyline (Carbonlite), a conventional poly boat, a 3-layer poly boat, and a fiberglass boat.
I like durability and repairability. You can buy many composite hulls for much less than an Eddyline. I have an Eddyline in the back yard purchased new in 08 I think it was.
Iāve had my fiberglass sit-inside sea kayak for 21 years now, and while it has scratches along the bottom from being dragged in sand, I havenāt had to do any repairs. Iām not super-careful with it (I drag it in sand as little as I reasonably can, but since I launch and retrieve at beaches unassisted, a little sand friction is often unavoidable).
I have never hit a rock at speed, which I think would probably cause damage that needed repair.
Iāve only ever used boat cleaner on it once, but thatās because I donāt mind the brownish waterline staining that slowly develops (itās resistant to milder cleaning products).
The gelcoat on the deck (top) is fading, I will say that. It should be buffed and a protective coating added (and renewed annually) to counteract sun damage. All the deck rigging I have makes that an unappealing prospect, so I am currently procrastinating on that job.
I do know that plastic kayaks are vulnerable to warping when stored improperly (for example with support points too close to the ends or too close to the middle). This doesnāt tend to happen to composite kayaks.
Thatās all I have thought of to address your question. Hope some of it helps!
I didnāt know there was bad sex. Less good perhaps.
Same with paddling . Iāve had some lousy trips but they all beat doing nothing.
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I had bad sex when I was like 19,you know, about 2 years after my peakā¦
Itās OK to go slow on buying gear. There really isnāt a need to gather a āfleetā if you are paddling from the same place and doing similar paddles. Most people who gather boats have them for different purposes, touring, surfing, whitewater. After a while folks start to have quiver for slightly different conditions, but it doesnāt sound like you are at that point. At some point most of your boats will go unused. From my experience composite seakayaks are very easy to fix and just as easy to damage, so itās sort of horses for courses as they say. My advice from years of buying and selling boats is find a couple you really like and maybe one to keep around to loan to visitors . Until you feel like your plastic boat is holding you back just be happy and donāt hoard boats as they come up for sale, since lots of folks want to be out there paddling too. Y
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SeaDart,
Very fair points made by you, one of the reasons I want to add is I have 3 kids and so far 2 of the 3 are loving getting out on the Bay. My fishing kayak is not 100% a great fit for the bay in rougher days so I am looking to have enough kayaks if all 3 and me want to go out plus a spare.
Iām a plastic kind of guy. About the only thing I take care of, boating wise, is a drysuit. I show them a bit of love. Everything else gets dragged, dropped, and beat on. A bunch of broken boats under my back porch. Too bad the car aināt got more plastic, got lots of scuffs on the roof and rack.
Around here it is the same. Rec-kayaks and a few poly canoes. They are considered semi disposable most live outside year round and to enter the river or exit there is normally a drag over an unfinished bank or at a few spots a drag down a concrete ramp. The river bottom is rocky and in places shallow enough to switch from paddle to pole.
Low cost and no one in a hurry.
There is a place for everything
Itās an interesting question. We have plastic, composite, and wooden kayaks. Over time weāve noticed that the plastic kayak is the most beat up even though we treat them all gently. The plastic has gouges out of the bottom and sides, and has faded even though we store it covered and treat it with 303 boat wash a couple times a year. Itās probably going to end up in a land fill some day. Meantime, the wooden boats hardly show any wear even though we use them a lot and they are pretty maintenance free. The composite boats get hardly visible scratches along the keels and are also pretty maintenance free. Most of our kayaking is done in Maine with launches from ledge and rocky beaches. On the water, there are some major differences. The plastic kayak, which was our first kayak (a Perception Tribute 12), is a slow tub compared to the rest of the fleet and we hardly use it anymore. Itās too heavy in the water, and tiring to paddle compared to the other kayaks. The other kayaks are a joy to paddle- easy to maneuver, quicker, track straighter, and since they are less tiring we find we can paddle farther and stay out longer. The wooden boats are a Chesapeake 17 and a Petral Play. The composite boats are an Eddyline Sitka LT and a Stellar 15 LV.