Most thieves aren't career criminals in search of wallets. Perhaps a respectable working man sees a wallet of a young rich kid currently in the water. The man could use some extra cash and sees large bills protruding from the wallet and decides to take it. Why not? The man has a family and that kid probably has a trust fund.
The wallet stuffed in a shoe or buried under a towel prevents temptation. It's not actually more secure but it keeps honest people honest.
Nothing is really secure. Adding security is all about making something harder. Harder to live with, harder to access, harder to do un-detected, etc. If hiding something from plain sight makes it even slightly more troublesome of a heist, it's actually security IMO.
It shouldn't be your only form of security but to write it off completely seems silly.