canon 7d is a geat camera imo. in low light, it does not offer as low noise at high iso as canon 5d series or 1d series. however, images turning out might not be great when quality of light is not there even with full frame. thus, number of people are choosing using flash (when shooting candid) or video light (when shooting portrait) in low light even with full frame, even pro... i am not a pro, just want to let you know. i am a curious hobbyist who willing to learn (NOT pay) anything to make a great image
i own 5d mark iii too, but still keep my canon 7d and canon 30d (mostly experiment and shooting my kids) for fun as well as backup, yet something that canon 5d mark iii cannot perform. for example: last weekend was my FIRST TIME of shooting bird. as if i did not have my canon 7d, i would probably not get any image or i would have to wait the bird get close to me.
in short, a crop mode camera is highly recommended if you do not own a long range tele-photo lenses:
here are some images that i shot last weekend with my friends. these are NOT that great, but hey first time of shooting. yet, i did not have flash, tripod (just camera and lens) since i do not have any experience about shooting bird. thing that i have learned after last shooting is flash duration...
note: these images has been cropped a lot, linked from facebook and shot with canon 7d, 70-200mm ii hand held... i am more about journalism than wild life