Wasn't it implied by either Scott or one of the writers that David was a combination of Weyland and Vickers?
Originally, the great Max von Sydow was going to play Weyland. That would have significantly reduced the movie's overall cheese factor and probably would have been much more effective.
If Fassbender had played Weyland, David, and Walter, I think it would have been too similar to Brent Spiner playing Soong, Lore, and Data on Star Trek. The dynamic between Walter and David was already a little too similar to the one between Data and Lore for my tastes.