Many here ask about a 7d in low light and before you know it someone recommends they get a 5D mark 3. Sure the 5D3 is great. It also costs 2.5 times the price of a 7d and wont isn't compatable with EF-S lenses, so often it really doesn't matter if the 5d will drive you to work, it is not usually an option. Fair enough if the question mentions budget as not being an issue etc etc.
The 7d has in fact Canons best crop sensor (shared with all their other crop cameras), so if thats not good enough people have to look at other brands or go ff, but to say it is poor in low light is not fully justified. 4 years ago it was probably the best on the market, so does that mean anything taken in low light with a crop previous to that is useless.
There are so many things you're overlooking in this rant. The most important is that what is right for you may not be right for someone else.
Personally, if low light is the biggest concern, I'd go for the 6D. It's got a slight edge over the 5D3. But other people have different opinions. Opinions are like armpits. Everyone has got a couple and many of them stink.