Its room dependant mostly. My room I don't have much room behind to spread out the surrounds to make use of the extra rears in 7.1. But I think the extra surrounds will become important as blu rays are starting to see more and more DTS HD MA 7.1 soundtracks and some of them have very aggressive mixes even in the surronds.
Could you do without the heights? Yes. But if your like me and have an extra set of bookshelfs laying around, a receiver with DSX capability, and no room for rear surrounds than its worth it IMO. I can't really tell where the height speakers are by listening but they do add to the soundstage especially for big blockbuster movies with loud soundtracks, Avengers, Transformers, 2012, Lord of the Rings ect...