All becomes are very limited compared to your own powers; sure you get access to a whole new set of temporary powers, and some of the becomes have more powers as you level up, but they're not as good as natively-picked powers. You can't add advantages, and you can't change the hotkeys (at least not permanently; they'll reset if you change back and then transform again.)
Also, most becomes are time-limited, meaning you'll eventually lose the ability to use them. Permanent becomes are worth your weight in global resources.
I was highly disappointed by the power armor become in the Harmon Labs alert, for instance - my Glacier AT tank had 8k HP due to scaling prior to using it. Afterwards, 5k, and I wound up dying to Warlord in like 15 seconds.