I've wondered the same thing: should a vehicle be abandoned as soon as it's immobilized, regardless of armor condition? In my opinion, any vehicle should have a high chance of bailing if it's got less than 5 points of armor on any location or some other critical damage effect. At that point, any further common Medium Laser or AC/5 hits will go internal, and the crew is likely to be annihilated one way or another, so getting out makes sense. When you've got 40 points of armor in every location, and are still capable of keeping the enemy at bay with your own big guns, bailing and trusting in the protection of that sturdy starched uniform and a pistol doesn't seem like such a great alternative.
Ideally, having a LOW chance to bail if immobilized (re-checked each turn that the unit takes damage), but increasing drastically as armor is depleted below a certain threshold, would give a more realistic indication of crew morale. I haven't checked to see if there's an official optional morale check already defined.
Also, I'd love to see some kind of optional threshold where an entire AI force would begin to fall back, rather than as an individual thing for crippled units. The current options seem to be an all-or-nothing choice between having the force as a whole fight to the last functioning unit, or else abruptly forcing an unrealistic cease-fire at some specific condition. If I'm "winning big" as a commander, I'm not about to let badly damaged enemy units simply walk, limp, or crawl away if I can help it, otherwise I'm probably going to end up fighting them again after they're repaired. If I'm losing, I'm (generally) not going to fight to the death, but I'm probably not going to be able to negotiate a cease-fire that's far more valuable to me than to them, so I'm going to have to conduct a fighting retreat, if possible. In my opinion, the AI should have to make the same decision, and withdraw whatever units it can under fire.