>> I think the reason is that some people use the AF-ON button and some people use the AE-L/AF-L button, but almost no one uses both.
If that were the case, all the members screaming for a camera "with an AF-On button" would just reprogram the Ae-L/AF-L button on a D600 or D7100. I'm pretty sure the people here that want "AF-On" want two buttons.
I want it because I want to shoot AF-C Focus Priority and I do not want to use the Fn or DOF buttons to lock focus. But there may be other reasons to need both.