Having an offset Reflex sight is actually not too bad.
I have an Aimpoint Micro on my USC/UMP conversion, and I also have a Leupold CQT that I was gonna use on it too (still deciding on that one though, its a heavy sucker!).
I actually liked putting the Aimpoint on the side rail and being able to tilt it to the side, but again, it adds a lot of weight for a light carbine. On a rifle I wouldn't see it as a problem at all though!