The issue with using tower speakers for surrounds is more about the positioning and alignment than anything. As a practical matter, it's a lot more difficult to elevate tower speakers above ear level (which is what Dolby recommends), and the furniture in the back of the room will block, absorb, and/or redirect the sound if they're in the way. Bookshelf speakers are just easier to position properly and they're small enough such that they won't be obstructed by furniture or other items common to the back of a typical room.

On the plus side, you can run all of your speakers (except for the center speaker) at full range, and I tend to prefer listening to multichannel music with all of the speakers at the same level (with most movies though, you're probably better off with the surrounds elevated -- Dolby recommends 2' above ear level, and in my experience I think about 1' above is fine).