Probably in Scarab. Yes it prevents the situation where we cover whole map with powerful missiles.
Mutog for example can attack indefinitely. 
And Aspida also has quite many attacks.
No, at least not yet. Probably in last DLC. Nevertheless vehicles even without upgrades are quite powerful… Of course 3 fully leveled soldiers have better power output, but still vehicle has better durability than any combination of 3 soldiers, and for example such vehicle is a lot better than 3 rookies. Vehicles don’t keep up with soldier progression, but they have their niche as @jrhebert mentioned. We will see how it will look in that DLC.
To add vehicles. Not to make them meaningful replacement for all solder combinations. Still this is one of the few (or only one?) tactical turn based games that has vehicles in repertoire.
Maybe I will mention their other use cases… Or maybe not. Please search for similar topics about “underpowered vehicles” or something similar. People there explained few times what they are good at.
Found it. Here it is probably from this point of discussion where Rainer explains things: