Doing as much as you can on your own is an important part of mathematics education. A lot of questions have a clear-cut answer that is right for them. For example, someone with little knowledge of a particular field might have a specific question about that field that someone who knew the area better would be able to answer. In that case, hints are pretty much useless. A lot of the time, however, the person asking the question could answer most of it on his own (I decided the sex of the questioner by tossing a coin) and has just got stuck somewhere. In that case, a hint is the best sort of answer as it lets the person asking the question think for themselves and gain a deeper understanding than a question would do on its own.
Between these two extremes, you have to use your judgement when answering a question. If you don't think that the person answering the question could answer it themselves, even with a hint, then don't just leave a hint. If you do, then leave a hint.
Note that leaving a good hint is harder and requires a deeper understanding of the subject than leaving an answer. It's also tempting to leave an answer in order to 'show off' your mathematical knowledge, though I think users of this site aren't particularly prone to that. I would find a really good hint more impressive anyway.
So, to answer your question: yes, I think that a lot of the time a hint is the best sort of answer and should absolutely be posted as one, rather than as a comment. Hint answers are one of your most powerful tools as an answerer.