Okay, this has been bugging me for long enough now ;)
One very frequent action in the forum is to quote parts of an earlier post, please add a button for that to the editor. Should just enclose the selected text in <blockquote> </blockquote>
See bug 634600 for that use case.
Oh, right, didn't see that. I guess this would still be useful to quote only parts or things that aren't posted in the specific thread, but the main use case is obviously already covered.