First of all, there's this thing in the military called (varying by service and nation, of course) "user level maintenance." The crew is expected to learn a lot of the basic mechanical workings of the vehicle, conduct preventative maintenance (i.e., checking fluid levels), and do a lot of the basic maintenance tasks like oil changes. The first time I ever personally ever changed a vehicle tire in my life was on a HMMWV in 2003, for example. The only time your vehicle goes back to the shop for the actual mechanics to work on is if there's an issue that the vehicle's user is incapable of fixing, either because of knowledge or equipment needs. And when the combat elements go out to fight, the workshop folks don't ride with them. They're too valuable -- they're the ones with specialty knowledge in fixing and recovering the vehicles, after all -- so they stay with the HQ and the logistics train following the actual combat elements. This leads into my second point. . .
Which is the concept of the "mobility kill." The term obviously implies being immobilized, but in practice it references a vehicle being disabled despite still technically being able to operate in some fashion on the field. Lots of armored vehicles, historically, were not completely destroyed and their crews killed a man. The vehicles would take hits and something would break: the gun breach is jammed or the tank threw a track or the engine caught fire. Whatever the case, even minor damage could potentially force a vehicle out of action. In such cases, it was commonplace for the crew to bail out of the vehicle and either fight on foot or beat feet for friendly lines. Whichever force held the battlefield would then either bring forward their workshops or tow disabled (but not destroyed) vehicles to the workshops, have the issues repaired, and put that vehicle back into the action. The outstanding example of this in my mind is always North Africa in WW2, where the British used captured Italian tanks and the Germans used some captured British tanks & trucks. At one point during Operation Crusader, the RAF refused to shoot up German supply columns because they had so many Allied vehicles in use that the pilots weren't convinced they were actually enemies! Getting back to BattleTech in particular, my point is that many of the critical hit results on combat vehicles constitute disabling hits or mobility kills. I'm not familiar with the force withdrawal rules so this may be covered by those, but I for one have been in games where a tank gets immobilized but the guns still work so I just let it sit there and menace anything that comes into its LOS. In real life, that crew would bail and hoof it most of the time and thus the tank would count as destroyed in-game.
For a tank to suffer damage, the crew to bail and attempt to repair that damage in combat while under fire is, I would think, an extremely rare situation. As in, "last stand if we don't hold this terrain feature the world ends" kind of rare.
