###Is accepting an answer useless? I know some people think differently, but I do believe accepting an answer is functional. Skimming for answers, I always check for the accepted answer as a first pick to check, simply because the answer is proven to work at least for OP. This is especially the case if I am not too familiar with the subject.
Is an accepted answer always a good one, let alone the best? Absolutely not, but the same counts for voting. I've seen quite a few (up)votes over time on utterly incorrect answers, still the voting system is a good indication in general on the quality of an answer.
###Do we ask people to accept?
It totally depends on the case and, even more importantly, in what way. I hate to see someone pushing OP to accept his or her answer, even before anyone has had a fair chance to post a second/third answer, or the quality of an answer is all but a proven fact.
I don't see anything wrong however in a polite request to consider accepting if OP tells you excited "Yay, this works fabulously, have been looking for this for ages", but obviously is not familiar with the fact that he can accept an answer. Nothing wrong with a polite "(only) IF the answer works for you and solves your issue, would you consider accepting..." in such a case.
If someone's profile shows he or she is familiar with accepting an answer though, I'd never ask for acceptance.