But the point is, it's computing time costs utility, and so it can't waste it on things that will not gain it enough utility.

If you consider 2x1x1 cube to have probability of 1/6 of landing on each side, you can still be VNM rational about that - then you won't be dutch booked, you'll lose money though because that cube is not a perfect die and you'll accept losing bets. Real world is like that, it doesn't give cookies for non-dutch-bookability, it gives cookies for correct predictions of what is actually going to happen.