Maybe the changelist will overall balance out in such a way that the game is more broken than it is now, who knows. But faster movespeed will permit more movement and survivability than otherwise leaving movement the same, simply because shooting faster people is more difficult and because you can move from cover to cover faster while doing more things, especially if acceleration is also increased (though it probably won't be). Regardless of whether the rest of the changelist is a good thing, free-flowing movement is something Halo 4 is in desperate need of, and that'll likely be even more true after the changes are applied.

I'm not arguing that dropping kill times and buffing movespeed is the way to go. If I were in charge, my approach would probably be to shrink the maps down, and make shooting more difficult until 5-shots are again something to feel good about doing. But here I'm just suggesting that within the context of Halo 4, this particular change at least makes sense.