Your uses for form shift are basically two fold.

"Oh no, the boss has become untargettable and I will lose GL! But wait! If I form shift into Couerl I can smack him with snap punch/demolish and keep GL up! Life is good and beautiful!"

And entering a fight already in a form. Generally Couerl allowing you to get off a couple more moves while in your peanut butter.

There's not much other use for it. That said, once you get used to using it for openers and boss invuln phases, it will be amongst your most missed abilities when you're doing 50/leveling/whatever roulettes and don't have access to it.