Iain,
Those discs spin at high enough speeds that you don't want one to fail structurally inside your drive. It's not the "OMG it'll asplode and kill you" sort of thing that is rumoured on the interwebs, but a flawed disc that fails will pretty well trash the inside of the drive, and that's more expensive to fix than just buying a new game.
How old is the disc? If you bought it recently, you might be able to get the store to exchange it for a new copy.
If not, you might be able to talk the manufacturer into sending you a replacement disc if you send then the flawed one, for cheaper than buying the game again.
-BP