To get the most out of the game you need storage space. At a minimum, you'll need a currency tab and unique tab, and if you engage with some of the other optional content you'll probably need the dedicated tab that holds the items related to that content. I love PoE, but the bank tabs are not cosmetic, or optional so you'll generally want to invest at least $20 to get the most important bank expansions.
Is it worth it? Absolutely! I just don't think it's honest to call the microtransactions 'cosmetic only'.
Ancient