It's optional, but it's an upgrade for levels and a couple of bosses (Fortress 2). It's a bummer that i have to choose between a better buster for the levels and 2-3 bosses and a better buster for bosses (fortress bosses don't have weaknesses aside from sigma 2) with no way to revert the choice. An upgrade should be an upgrade. You also need it for the hadouken.
I like the game, I care deeply about this game and this franchise, and because I care about it i'm constructive because i want it to be the best it can be.