Lots of good feedback here so far, but wanted to add a couple of thoughts.
If you think you want a 2nd body, you probably won't be happy until you get one. I've had a 2nd body ever since I bought my 2nd dSLR: I keep the previous body as my backup. Currently, a 5D3 is my primary, and a 5D (classic) is the backup. I shot with the 5D for 5½ years before getting the 5D3, and I haven't had any problems switching back and forth: the 5D3 is easier to use, but the 5D mechanics are ingrained in my memory.
I only pull the backup out for a couple of applications: 1) I'm doing a professional shoot and I need redundant everything, 2) I'm hiking and don't want to be constantly changing lenses out in the elements. Fortunately, my wife is always with me on the trails, so she helps with handling the gear.
Secondly, I agree wholeheartedly with J.R.: if you don't have a flash, you don't know what you're missing!
I can certainly appreciate shooting in low light when flash is prohibited, undesirable or for strictly artistic purposes, but if you want / "need" a 2nd body with good low-light AF just because you don't have any flashes, a speedlite is much cheaper than a 2nd body. So are 2, 3, even 4 speedlites (and getting into off-camera flash is where it really gets interesting / creative / fun!).
I have 4 600RT's and if forced to choose, I'd keep them (at least 2 or 3 of them) before holding on to my 2nd camera body.