I've done towers for my SL/SR...I have a young child in the house and having a tower is much more stable than a bookshelf on a stand.
Having said that, the vast majority of source material doesn't take advantage of a tower for the rear, ie, not too much signal being sent through the surround channels in movies etc.
Useful for 5.1 SACD, DVD audio or '5 ch' listening, but some people prefer the '2 ch stereo sound from the front' effect vs the immersive all tower setup.