>>2018079
Not all of them evolve, I think there are less that do than don't, but the ubiquitous elemental units might make it seem like that's not the case.
I also wouldn't necessarily say expendable as much as easily replaceable, since if you happen to lose one you can simply drop a new one into your army regardless of how far they are away from home (as long as the army has a hero of course). Mystic summoners also have the ability to just flat out buy ranks with their special resource, lessening or even eliminating the blow of losing a ranked up unit.