I have a jar at home full of leftover currencies. Lots of European (pre-Euro) ones. They're just souvenirs at this point. Probably no more than a couple bucks' worth of any single currency and I'm sure I've missed the final convert-to-Euros date on some (all?) of them.

I can see this being useful if I had $20-40 worth of currency. Big enough that it's worth my time to convert; small enough that a no-commission service with a worse rate is okay.

I have leftover $20-40 worth of GBP, EUR, and CAD on purpose - just like to have it for my next trip. Sometimes helpful upon arrival somewhere if an airport transit kiosk won't take a credit card or whatever.