For people buying as a backup:
Even if you got $300 deal, you will need to plunk couple 100's for accessories to be able to make it usable to the level that you have 1 shot every 3-5 seconds or more!!!!
Much better to buy a used 60D instead.
Now, I am not going to bother to spend any dime on accessories at all.
IMO, its oxymoron, as with anything except pancake M lenses, you loose for form factor. So what are you achieving by plunking $$$, a seriously crippled 70D alternative for same price and form?
While it may required the extra purchase of the EOS-M/EF adapter (not a couple hundred, by the way, but a substantial cost), the EOS-M fits some people's idea of a backup very well where a 60D would not.
If I'm carrying my 7D and several lenses already, it is much easier for me to toss an EOS-M in my bag (with adapter) than it is to carry a 60D. It is a smaller camera.
Backup in this case answers the question "what is small enough to carry with me that allows me to still use my lenses if something happens to my 7D." I can carry an EOS-M and adapter with me more easily than a 60D.
To paraphrase from the "the best camera is the one you have with you" crowd, the "best backup is the one you have with you." For me, a 60D would not fit that criteria.
To counter the above statement, however, I do consider the SL1 to be acceptable in the same capacity. I have carried both the EOS-M and SL1 as a second body. Both are small and light.
As a second body, and not technically a backup, I also find it useful to carry one of these in addition to the 7D so that I can have two different configurations set up. For example, I have on numerous occasions used one body to shoot video from a tripod while I am free to shoot stills with the other body (you can hear my mirror/shutter noises on the video but that's not a problem for my use). Again, they are much easier to carry.
We always need to remember that things like backup do not mean the same thing to everyone.