That's a very good point about their utility not being realised until max level, Khmr Princess is a great example of this - triage added at Max, and no use until diamond league with a max summoner which I don't have any of.
I play mostly in Gold, tend to get up to Diamond 2 at which point I seem to come up against legendary summoners all the time, so what's the point?
As to the untamed cards being underpowered, you might have a point - nearly all of my go to cards are Betas. Untamed seem to be useful for replacing individual cards - the new dude at 3 mana in life for protect rather than the 4 mana dude with magic, and Kmr could replace the wood mymph - but there is a lot of fluff in there, some of the reward cards are more useful.
Defo add ons to Betas.