By having characters locked until you’ve finished particular objectives you are having players play hunters/monsters they may not enjoy before they can play hunters/monsters they do. I don’t see a benefit to doing this short of some feeling of progression, but that feeling of progression can still be felt in the buffs given to the weapons with each star. Only other thing I could imagine is that tier one characters should be easier to play than the others, but there is still nothing stopping anyone from playing said tier 1 characters. Also, my ability to understand Maggie’s kit wouldn’t really help me in knowing how to utilize Abe’s kit as they only thing they share is class (and dome) and I’d have to learn that role regardless of who I was playing to begin with.
In the grand scheme of things, by having all characters unlocked you would be able to play whoever you like and experiment with team builds right away without having to play characters who do not interest you or whose kits you may not agree with.