Currently, destroying enemy units is somewhat unrewarding. By destroying them you DO have less active threats to fight against and this is the only thing that's stopping you from rolling right into their base and capturing it -- but this formula's a little long in the tooth and becomes slightly tedious after a short while. What if you could create more incentive to destroy enemy units and reward players for doing so -- and at the same time create more attachment to a specific unit, increasing it's value to the player emotionally even if it doesn't affect the game balance?
1- Proposal: allow units to gain veterancy. By killing enemy units, your specific units can acquire prestige, skill, valor, rank, whatever you want to call it -- a measure of how much effective combat experience they have. You could progress a unit through newbie rank, to skilled, through veteran, and maybe in exceptional cases, heroic status.
What are the rewards? Well, offering cool unit rank insignias would be cheap to do -- provide an icon next to their number on their "stat card" on the carrier. Provide a little drum roll and "level-up" notification with some text when the vehicle levels to let the player know he's doing a great job. On the list of lower priority (more expensive) things to do, perhaps the unit AI would get better at aiming and dealing damage (or moving!) when the player isn't directly controlling it. Maybe there's some other perks, like a fire extinguisher that puts out fires (i.e. repairs that last 10% of your hit points back up to 20% if undamaged for a period of time), or maybe there's nothing besides the icon and the notification. Might be a cheap and fun reward to be able to "name" heroic units -- that could be an ultimate reward.
This could increase player attachment to units, and make them play more carefully with some of their resources. You might think twice about sending your entire squad of walruses in on a risky objective if one of them was of heroic status -- don't want to lose "The Crusher" (my named unit...)
2- Proposal: A completely different alternative could be to cause enemy units to drop a small bit of health, ammo, or energy. Collecting pickups for killing enemies is addictive and fun -- if it seems "outside the fiction" (I'd argue the fiction is slightly tenuous already, but whatever ) perhaps vehicle corpses could be "Scavenged" for small rewards. This wouldn't put off the eventual return to the carrier for resupply and refit, but it could delay that long (usually boring) process.
Thanks for taking the time to read this. I know I'm not an expert on your game, and have no doubt that you've got a long list of features you want to see developed yourselves.