I want to be able to look left, right and behind my vehicle, even if I can't shoot in that direction I'm looking. This would really help when backing up to ensure I don't run into someone or something.
Now as for Hat up, perhaps it should be to look down instead like I think I recall it was in MW4, sometimes can be very useful for when you're jumping your mech over something, or if you're in an aerospace you can see if your bomb hit the target or if you were off.