In some cases though, the registrar only pays the registry for 1 year at a time, making the money their customers pre-pay effectively a low-interest loan for the registrar. If that registrar goes bust, you'll be able to transfer your domain to a different registrar, but you'll lose the money you paid (i.e. loaned) to the original registrar.

There's also another risk: if you decide after 2 years you're not happy with your registrar, tough luck, you've already paid for 10 years.