Doesn't carrying spare mags OWB (unless it is a discrete pouch like one that looks like a knife pouch), kinda defeat the purpose of CC?
It depends. Many people can conceal a handgun in an OWB holster. Under a concealing garment, even if the mag pouch prints, it'll look like a cell phone holder, or knife holder. However, I have an OWB mag pouch that conceals very well, under nothing more than a T-shirt. I've already mentioned it in this thread, the Dale Fricke horizontal mag pouch.
Obviously if you're wearing a tucked shirt, it's not going to work, but in that case most traditional methods of concealment won't work anyway, unless you have a coat or vest on over, in which case, this mag pouch would still work.