Space rescue isn't great, it's downright awful. Ejection is out, unless the seats are fitted with some pretty extensive manually controlled thrusters or vectoring nozzles. That means the fighter needs to be intact enough for you to bail (not great), and then in some way halt your momentum. And in aero combat, that momentum can be pretty hefty in space. Then, SAR craft need to pick out your beacon from amidst everything else going. I don't even want to think about the often used sci-fi idea of SAR being done during combat. The level of tech needed for reliable space based SAR exists in BT, but not in the numbers or even in the type of craft needed. Small craft in canon material are a black hole of gaping omissions in the TO&E needed for strong combined black water combat ops.
Atmospheric ejection should definitely be fine. But there's a whole lot of NOE and low-altitude flying going on in Battletech. Ground combat, that's where the action is, so that's where the fighters are. And low-altitude and ejection equals crimson mist way more often than pilots will like to consider. Also (and I'm only judging from my own experiences with aerospace combat, which is honestly limited), aerofighters tend to die ugly deaths. There's rarely ever salvage, and almost never in atmospheric combat (the case could be made for low velocity crash landings allowing for some degree of salvage). And the aero battles I've dealt with in atmosphere have typically ended with easily critted/heavily damaged craft flying away, getting lawn darted, or trouncing the ground forces. There's very little in between from my experiences.
