Expense is the problem. For a while, I was full of desire for card based equipment for PCs, stack item cards on your Back Pack Card, Weapons and such on your Belt Card and so on. It didn't take long for me to realise the amount of work and expense involved for something that could otherwise be solved by writing it down. Helper Cards are a great idea, but the execution of that idea will be problematic and could potentially place artificial limits on your imagination through the standardisation of the items, creatures and everything else encountered in the game world. Nice design, though.