I have speakers hooked up to the "presence" channels of my 2400 and I really like what it does for HT. It seems to lift the whole sound stage and it gives the impression of a much larger theater much like you would hear in a movie theater. After listening with the presence channels for a few months I tried it with out them and really prefer them in the system. I would suggest trying it before you buy new speakers just for presence channels. Perhaps you could borrow a pair of small speakers to check it out with.
I've been reluctant to add rear speakers because there doesn't seem to be a heck of a lot of material out there that supports it yet
*Does your train of thought have a caboose?*