One photo for HDR?

The usual process is to take at least 3 photos at different exposures and then combine in e.g. Photoshop or Photomatix. Unless the subject is absolutely static, I always get ghosting, even when using Canon's auto 3 shot with one click process.

Why not just take one photo in RAW, make 2 copies and adjust the exposure in a RAW processor to +1 and -! and then combine the 3? One pic, no ghosting. I've used this on a couple of occasions but I suppose it can't be that simple. I'm guessing exposure correction in RAW is not quite the same as "when taken". I suppose a single shot can have 0 in the shade and 255 in the light and so no correction would be possible in PP. I think it would work however if the single shot retained information at the extremes.
--
Descartes to his wife - "You annoy me, therefore I exist."