Of course being able to roll on one side (only) is much better than not being able to roll at all, and in that sense is competent rolling for a kayaker. However, that's not the question I'm asking.
The question is if you want to consider yourself a competent roller in all reasonable circumstances, a highly skilled roller, an advanced intermediate roller -- I'm not sure what words to use -- do you really have to have a roll on both sides?
To ask the question a different way: If you have a solid roll on your strong side, should you be content with that? Or should you continue continue your practice onto your weak side as a natural progression of kayaking competence?
If your answers would be different in ocean vs. whitewater vs. lake paddling, I think that would be of interest to the discussion.
(My question arises out of another paddling symmetry issue. I argue that a really competent single blader should be close to ambidextrous with all strokes under all circumstances, but I think that is a minority view.)