I think I get the objection, less on the basis of 'more stuff to carry', but more on the basis of "if the key is generated on the level it is used, it just slows you down to run over, pick it up, and run back to the door".
I'd actually be more in favour of the idea of small locked rooms generated with extra treasure that require keys, when I stop and think about it - ultimately giving the players a choice if they want to cart a card key around with them or not. Kind of like carrying around an Envirosuit pack "just in case" you happen to need to cross a room filled with magma, players could opt for carrying a keycard "just in case" a locked vault spawns.