common sense says the Oyster should live in the flimsy
plastic wallet separately from all my cash and credit
cards ...
Common sense? For what reason? I would find it inconvenient to have
two wallets.

Getting your wallet out with all your money and credit cards when someone
could snatch it and leave you the wrong side of a gate line. I too find it
convenient to keep them all together but it's crossed my mind it's maybe not
the best thing to do.