For movies most of the time the subs plus limited-LF surrounds and rears are (were) fine. But some movies have scenes with things like explosions, aircraft (fixed or rotary wing), spaceships, or what have you that are panned around the room and those are IME/IMO much more realistic with good full('ish)-range speakers all around. For music, again some mixes just include some reverb and audience noise, but by and large I've found multichannel music often seems to put more and deeper bass in the surrounds than many movies. And as Rex noted the footprint of many towers is about the same as bookshelves on stands, and the occasional loud effects in the sides and backs lose much of their impact when little speakers bottom out or all you get is "bip" instead of "boom".
I started with smaller surrounds and rears (Mirage Omnisats), went to towers (KEF and Infinity at different times), then back to limited-LF (Magnepan MC-1) over the course of perhaps 10-15 years. After living with the Maggies (my main love speaker-wise) I realized I loved the sound but really wanted more bass from the surrounds (rated to 80 Hz, but more realistically 100-120 Hz as measured in my room). Changing to floorstanders (F206's initially) was a significant change. I was able to drop the crossover a little, from about 90 Hz to 80 Hz, but there was a big change in perceived (and measured) bass with the towers. The bottom end was filled in sooo much better and the sound more immersive. I understood the change I would be making, but even so, the level of improvement caught me off-guard.
The only problem I had was that my couch was high and I needed to raise the floorstanders a little. After piddling for a year or so I finally just bought some big concrete blocks and painted them black. I had barely gotten them under the rears when a good deal came up on some blems and I replaced the F206's on the sides and rear with speakers standing a little taller. The solution is working out extremely well, but admittedly using Salon2's for surrounds and rears takes a bit more area (and budget) than using F206's. Anything worth doing, is worth overdoing, I always say...
Blame John and Rex, plus some emails with Floyd (con, voted for Gem2s placed higher) and Kevin (pro, using Studio2s in his system), for the change. I placed the sides a little in front of the MLP, but I have done than more often than not over the years, both due to space constraints and because I actually like the sound stage better that way. Yes, being contrary, but that's me.
I do have a fair amount of experience using smaller speakers as the mains dating back to the 60's or 70's. I can safely say that every time I have upgraded to towers I have been much happier with the sound, and every time I have gone to smaller mains I have regretted it and longed to go back to larger mains. That bias undoubtedly contributed to my desire for large surrounds and rears, but I am happy with them. And the Revel cabinets are really gorgeous, enough so that I've a house full of them now and my wife likes them (a very tolerant soul who also appreciates good sound -- she plays piano and organ whilst I sometimes honk along).
FWIWFM - Don
p.s. For those with small, or big, kids, note floorstanders are usually more stable and harder to knock over than bookshelves. Just sayin'...