They thought that improving your characters should never give you a disadvantage, and when they changed GAC they made it very clear
I should add that they ALSO made it clear when they nuked the Paper Zombie strategy for NS (A g1/L1 self-resurrecting tank on a squad that gains TM when allies die was MUCH better than a g12 self-resurrecting tank who wouldn't die as often).
The rework of Zombie to make sure the toon granted more advantages at g12 than at g1 also came with a statement that they were committed to making sure no one ever gained an advantage by not improving toons. So that's twice that they told the player base in no uncertain terms that this was going to be important to their design philosophy.