It happens frequently (at least to me) that I write answers to questions and the original asker not only doesn't mark my answer as answer (which is reasonable) but also doesn't comment at all, giving me the idea that his question was not so important and the time I used to answer is question was a waste of time.
Sometimes answering questions is an activity that takes time: I think this effort should be respected by the original asker, who at least should mark the answer as "read" to give the minimum satisfaction to the ones who answers.
A flag marking the answer as "read" without the need to comment further, at least to let the answerer know that what he did was read.
Thanks for your time to read this (I'll read all the answers :-) ).