Why don't you like this feature?
I find that especially in some extremely "hot" threads it's more trouble then it's worth, and of little to no worth in "normal" threads...
Making this another option would IMHO just complicate things even more for users.
Well, I don't think it's that complicated...
I don't know phpBB's design choices (if any) on this point, but in general leaving options out because it "complicates things for the user" is a bad thing IMHO ... If software has lots of "advanced options" you can always make an "advanced tab", or "advanced mode" or something along those lines.
Another thing that phpBB currently doesn't do is offer help for it's options, if you look at (for example) vBulletin's usercp then each option has one or two lines of description, obviously making things easier for the user.
But these things are a different (larger) discussion...
UNIX was not designed to stop you from doing stupid things, because that would also stop you from doing clever things.