There are several valid reasons to make your feedback private, usually it's a temporary thing while you work to get an obscene or otherwise violating feedback comment removed.

It used to be much more common before feedback went to "transactional only" early this year. I once saw a seller get "bombed" with over 200 negatives from about 4 usernames in one evening, he obviously went "private" until he could get them removed.

Other people, (usually sellers of "adult" items) make their feedback private so that their customers can have some privacy.

I would never bid on an item offered by a seller who had private feedback, but oddly enough, I would have no problem selling to a buyer who's feedback was not viewable. I mean, I'm going to get the money before I ship, so what have I got to lose?