... and force all your friends to upgrade as well? Facetime makes even less sense when only 50% of your friends own an iPhone and only 50% of those have the iPhone 4S/5. Why wouldn't Apple allow this on all iOS 6 devices? This and a stable voice-only option could finally make normal costly phone calls expendable.
Battery concerns maybe?
FaceTime is a novelty for me, I used it once when I first got my iPhone 4 two years ago and haven't launched it since.