If you are a person who adds mods to their minecraft, you will know when a new version of minecraft comes out, you basically need to delete your .minecraft folder, or the bin folder itself. This makes it so your achievements are bye-bye. The achievements are specifically useless if you just keep losing them. If they were saved to your account though, then as more achievements are added to the game, you could see which ones you need to complete, you couldnt ever lose them again!
AWWWWWWWWWWWWWWWWWWWWWW YEAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!
But yeah , if i some day i decided to care about achievements , it would be a good idea.
Take some porkchop