At the very least, it's worth having dealings with multiple banks (eg your savings account with one and a credit card with another). It wouldn't take a financial collapse to make your money temporarily vanish -- I mean, how long was Playstation Network down for? It's entirely conceivable that one day a major bank will have a serious computer problem that'll shut them down for days.

Or failing that: heck, what happens if you lose your wallet? Yeah, a few hundred bucks (or quid) in a piggy bank isn't a bad idea.