The legacy HUDs lack the proximity indicator (doesn't seem to be explained in the game guide but Colin mentioned it in at least one of his interviews) of the HD HUD.

You can use it to:
  • judge when somebody is sneaking up on you (especially useful when you're in first place)
  • block (non-turbo) passing attempts (as long you don't expose yourself to forward facing weapons)
  • perfectly align mine and bomb drops in front of the craft behind you
  • judge when to start a pulse to (in most cases) eliminate the passing craft