Quote from Savages
The real thing they are trying to accomplish here is to give a disincentive to skill swapping. It's not to make it so that you don't have to have the MF stat on your gear.
They're totally trying to disincentivize skill swapping, true, but that's not the only thing. Motivating people to sweep through the game and kill champions/bosses alike is huge. It completely changes the endgame.
I like the idea of the valor buff stacking with infinite duration, but the problems are pretty easy to see. Resetting the buff at the start of each new act seems like a cool solution, you would pick which act to run partially based on length, and try to make it through the whole thing in a sitting. It's fine if you don't, but if you do you get better drops/makes the end boss of the act that much more epic (because you'll have high valor stacks).
It also makes runs a 2 - 3 hour endeavor if you want to go through the whole act, which fits pretty well with your typical Diablo player's play session I think. For those who would play longer there's a nice breakpoint for devouring food, and if you play in hour chunks you could still farm pretty effectively (though unfortunately not quite as well).