If a collision is unavoidable, you want to hit the other vehicle with the beefiest parts... the bumpers, not relatively flimsy sheetmetal above it, regardless of the armor added. Typically going high works better, assuming you are trying to avoid flipping it. Going low suffers risking the other vehicle hitting the cage directly and also having some crush damage... could the attacked vehicle continue driving if the attacking vehicle was parked on top of it? In any case, crouching while driving would cause the vehicle to drag on the ground, risking losing control.
Crouching would not stop a forklift or bulldozer from getting under the vehicle, as the seal would never be complete. I imagine plenty of things could still be thrown under a crouched car... it is simply better to drive away from threats like that.
Which brings my to the first rule of car security: keep moving... crouching is not moving. A vehicle stopped will eventually be compromised regardless of armor.
My final point about crouching is that it is hardly a good security feature if it can't be immediately uncrouched... but that's just my armchair quarterback's opinion of the situation.
I'm going to stop there before the black helicopters are dispatched.
It's not a forktruck that Limo 1 is concerned about., it can outrun even the fastest of fork trucks. It's propelled explosives getting under there that they want to avoid.
Crouch, survive rocket attack, un-crouch, run.
The car can withstand a rocket attack, but I'm sure that it's underbelly it it's weakest point and is also a way for an explosive to flip the car (then it really can't run).
I doubt it uses air shocks... too vulnerable to failure. Most likely it's hydraulics. It can be raised or lowered in either direction, and fairly quickly at that.