Your answer might be trivial to you, but if the question does not have a selected answer, then what you have to say could be very valuable to the person who posed the question, and if not them, someone searching and READING the question...
I've answered questions that are MONTHS old, and still got accepted answers.
Or, maybe you shouldn't.. Give someone like me a chance to answer... heh... Just kidding..
I really think that if you have something to add to the question, or the information presented in the thread, you SHOULD in fact answer. Even if people got the answer, but didn't MARK it, someone else might come along and read your answer and get valuable information out of it... I know I would.