I just think people are overusing it. If you're Kentucky and you think you've got an offensive mastermind at OC (apparently not true, but when they named him head coach in waiting, maybe that was a reasonable guess), you know you're going to lose your coach soon, it makes sense to name a head coach in waiting if it lets you keep a good coordinator that will likely be your best candidate for the HC job anyway.
I don't know why the hell Texas would use it. If you're 17ing Texas and an assistant wants to leave, you hire a new one and when you need a HC get the best coach available. Doesn't make sense for FSU either. Let Bowden fail and you should be a desirable coaching spot. Florida recruiting base, competing against the ACC, and I'm assuming good money.
But if you're a lower tier SEC team or middle tier in any other BCS conference, I can see it. If Byrne is as high on Hudspeth as everyone thinks, I could even see an unofficial coach in waiting title being offered to him; although the ninja might would rather just wait until Mullen leaves, look into the future, and pick the best assistant available.