I currently use a pair of 7Ds and a 5DII together. Earlier I used (and still have as backup) 50Ds alongside 5DII.
I'm sure there's someone out there who uses one of the Rebels, perhaps as a cheaper body primarily for use on long telephotos or as a lighter alternative when they don't want to haul around a larger kit. The IQ of the latest Rebel 18MP models is virtually identical to the IQ of the 7D (and 60D).
Personally the main reason I'd be reluctant to use the two together is the control layout of the Rebel series is so different. If switching back and forth a lot and/or quickly, quite different controls might cause you to make errors or pause while trying to remember which is which.
I never actually had a problem switching back and forth between the 5DII and 50D, but did worry that the image erase button on the 5DII and image replay button on 50D are in the same location, and vice versa. (I use the "set" button for image playback on my cameras, so this really was a non-issue for me.) The other control that sometimes gave me pause was the top LCD illumination button, which is on opposite ends of the top button array on 5DII and 50D. 7D and 5DII are more similarly laid out, except of course the 7D's are more customizable and it has some additional controls for the more sophisticated AF system.
Also, 7D and 5DII share the same memory cards (CF), batteries (LP-E6) and chargers. 50D uses a different battery (BP511/511A) and charger. Rebels use different batteries (LLP-E8), chargers and memory cards (SD). 60D shares battery/charger with 5DII, but uses different memory (SD). 5DIII uses both types of memory (dual slot: CF & SD), and same battery/charger as 60D, 7D and 5DII.