Across StackExchange, I quite often use the commenting function when I want to add something that I feel may well be relevant to the poster, but doesn't answer his question (as asked) so will probably not be too relevant for future visitors.
However, recently, such a comment was removed from this site by a mod, which makes me feel this is not accepted commenting behavior on this site. I am not quite sure what to do if this comes up again. Let me expand on the example, as that will probably make it clearer.
The question was What to do when players bypass plot hooks and the poster described how the plot hook he was having trouble with involved a drug user. I, then, commented on drug use with a small comment that I felt would possibly help him with this very plot hook he was having trouble with. However, it never attempted to deal with the larger issue or the general question, which was about plot hooks.
My comment (along with a number of other comments) was removed and replaced with this comment:
Please don't leave "lite" answers in the comments. If you feel you have material that should be preserved for the ages, either make a comment on an answer suggesting its addition or forge it into a proper answer — even a short one.
Another (meta) example would be Should the metric system be avoided when referring D&D 3.5? where I would like to add a note explaining that an edit does not mean the original was wrong or should be avoided. It does not answer the question, nor request an edit to the question, but it does help the general understanding of the situation.
In neither case I tried to answer the question, so it wasn't an answer. It wasn't something that should have been in the question, so it wasn't a request for an edit. Then, should I not try to be helpful in situations like this?