I see, so transaction fee wise multiple keys/inputs are a non-issue.

But doesn't this cause still dificulties when you for instance want to - for instance -  switch wallets?
I imagine the amount of keys in your wallet will go up constantly (if you have an additional pair everytime you receive coins). And then, when you maybe want to switch wallets you'd have to export all those keypairs and import them to your new wallet. Unless the two wallets (from different developers) use the same data interchange notation (which is unikely imho) youre looking at a lot of effort up to the point where you are effectively locked into a wallet.