Name one reason to wear a band t-shirt other than social pressure or narcissism?

Your argument isn't an argument against suits, it applies to any item of clothing other than the absolutely most basic, utilitarian items. It's an argument against fashion and self expression through clothing of all sorts.

Now if this is what you genuinely believe then that's fine but I hope you're applying the same standards you apply to those wearing suits to people wearing band t-shirts, anything branded where an equivalent non-branded item is available, or frankly anything that has been chosen for any reason at all other than it's absolute functional value.