Spidey’s purple is broken, it reduces strike tiles even when he’s not invisible and no ones stunned, makes him much more powerful than he really should be.
That’s the whole point of BSSM - reduce strike tile damage.
He doesn’t have to be invisible for that, the passive is not conditional.
Invisibility is independent from strike tile reduction.
Seems to work just fine to me. 
He is a very powerful character who has the ability to really weaken two of the most frequently used 5* characters and forces opponents to consider different strategies – that’s a good thing.