Is it an 80:20 split. I always thought that there was no power to the back unless it needed it.
As for side effects, I guess if you ever put your front wheels on something slippy, then it'll be trying to divert power to the backwheels, via fresh air
I don't think that the shaft will take out the box. Pretty much all the cases I've seen, it's the rear bearing that's gone, so the front half of the shaft is supported normally. The shaft itself still seems to spin freely even when the bearing is shot (or it did in my case)