By "default" doesn't mean right from the start, but once you can recruit units (when you've reached your first rank, at 5,000 points), you can then also recruit a machine-gunner (in addition to the AT-soldier or medic).
I added that option since you could easily equip any of the other AI units with a machine gun anyway, so this is just saving you a bit of hassle of having them change their equipment.
As far as the respawning is concerned - that has to be handled quite different from the MP version, since there you could theoretically have multiple cities active at the same time. Something you don't really have to worry about too much in SP.
In SP, on the other hand, you will often leave the city (to go back to the base), and when you return it's just nicer to find it in the same state as when you left it (that way you can actually execute that "cunning plan" you had when you quickly went back to the base to get supplies to reinforcements).
Oh...okay. I understand now...thanks for the response. I guess I'd better practice some more then.
