I don't think its a good idea to evolve the game mechanics around punishing players who throw away units (me), its 1 strategy to acheive high level units, but it shouldn't (in my humble opinion) be the only strategy, you still gain hp, accuracy and spell resists (and dodge if you picked that trait) for each unit level.

What I would like to see was a few more progressive traits though, that would be neat, so I could design a unit around being "babysat" until its level 10

Sincerely
~  Kongdej