I use a Descent with the Summits and though it doesn't always contribute as much weight to the presentation because the Summits go down very low, it has helped me to even out a couple of bumps in my rooms frequency response.
By rolling the Summits off a touch, I was able to get a smoother and tighter low end in the room when we did the measurements with the Descent in place than without. It did take a fair amount of screwing around and I do have a dedicated room, so had the luxury of putting the sub where it worked the best not having to worry about making it work with the rooms aesthetics.
Again, we are not talking about a huge difference here, so it will depend on your room and as the others mentioned, your choice of music. If you only have limited placement options for your summits and descent, it might not solve any problems for you in a 2ch situation.
I like the extra weight myself.