Grevious was supposed to be like darth vader, only he was so f'd up that he was more droid and the droid body kept him alive. GL and co. didn't want him to be exactly like Darth Vader, but somewhat like him. I guess they ran out of ideas for a new villian.
And Stephen Speilberg did a lot of constructive critiscizm to GL, "" His ideas were a little weird.
The deleted PT scenes weren't that interesting since most of it was Computer generated and unfinished.