When I look to make a trade with someone I look for two things: a decent quantity of feedback left and the percentage value that reflects the good/neutral/bad feedback combination. If someone is at 50 total feedback or below I try to see if they've at least been trading recently. If the percentage is anything other than 100%, I try to see what the one or two (or, ulp, more) non-positive comments were about before deciding on going through with things.
For some people, leaving feedback on their profile is important. They either need to get their totals up to add some non-newbie trust in the trading community and/or up their trade result percentage in case one unfortunate event is defining them. Therefore, leaving feedback is a good thing to do.
However, there are many who are 100+ positive feedbacks and are currently at 100%. Is that one extra positive feedback going to matter much? No. However, it would at the very least give a heads up to everyone else that the trader is still trading if someone were to look at the dates from the feedback comments. So, it's still good to leave feedback for these veterans.
Whether one is a newbie, a veteran, a squeaky clean trader or someone who has had a bump in the road at some point, it seems that getting even a 'Good trade!' can be beneficial for anyone's feedback profile. Taking that last step to leave feedback during the finalization of a trade may not only help the receiver of the feedback but also those looking at the receiver's feedback later on, especially if the comment is "Bad trade, because...".