- There is no specific benefit IF you put them in the Support or Rear line positions. In Front Line attack or Front Line Defense, however, Medium Bombardment can be useful. First off though, if you set the Artillery or AA to "Avoid Combat" they'll take an 80% malus to their accuracy along with an 80% bonus to their evasion. So it's not an ideal arrangement to say the least.
All Bombardment types EXCEPT Light Bombardment fire in a separate Bombardment phase. In the C# Ground Combat, you also have counter-battery fire. If you include Medium Bombardment vehicles in your Front Line Attack or Front Line Defense positions, they will fire on any enemy formations in the Support position that fired a bombardment at them, such as Light Bombardment.
- At least, they SHOULD do that; I still have yet to test this out for myself. As for vehicular Artillery and AA in general, vehicles have more armor and hit points than Infantry OR Static, but much, MUCH lower Fortification. That includes both Max Fortification AND Self Fortification. In terms of combat rules, this means they are decidedly worse than static or infantry, but with a big pair of caveats. The first big caveat is that it assumes your infantry and/or static units are allowed to get fortified. If the enemy is attacking you while your invading their planet... well that could be a problem. It is worth noting that you need Construction Vehicles to get to Max Fortification with anything anyways, so that's a factor too.
- The second caveat is that if/when your units DO get hit, they'll have more armor AND hit points, which is a bigger deal than it might seem at first glance. Penetration values of enemy Artillery, hell, ANY artillery tend to be meaningfully lower than equivalent tech armor, while the damage they do per individual shot isn't
that good. So your hit points will go further since you can more easily tank their shots, and if you're under fire during say, a contested landing, that will help... IMMENSELY. As for AA, Ground Support Fighters are underwhelming, so go nuts and do whatever you want. I recommend a Medium Vehicle with Light AA, Light Bombardment and Medium Vehicle Armor for Support Line and a Medium Vehicle with Heavy AA, Heavy Bombardment and Medium Vehicle Armor for Rear Line. It's worked a treat for me thus far.