I make this request for two reasons:
First: I find it rather annoying to answer someone's question and be thanked in a comment, while not getting the answer acceptance.
Second: It's also annoying to click on an interesting question, that apparently has not been answered well enough as to satisfy the asker, only to find out (by reading the comments) that the asker is in fact already satisfied with the existing answers, but simply not accepted any.
In my experience, most of the times this happens when the asker is new around; let's remind them to (or let them know they can/should) accept a helpful answer. Let's put a nice pop-up, or a simple legend, as a reminder of that, maybe when we are writing the question or right after posting it.