Fear cuts both ways. This question covers why none of the Death Eaters came back.
But if you're asking about Peter in particular, there's the manner of his death. It may have been different in the movie, but in the book, because he hesitates to kill Harry, the hand given to him by Voldie chokes him to death.
Now, this was a sort of booby trap laid in place should he ever betray/think of betraying Voldie. Would you really want to come back and face 'ol no-nose after that? In the powerless form of a ghost? Were I a consummate coward, I would most certainly chose the unknowns of death over facing Lord Moldiewarts.
The linked question above covers how powerless ghosts are, relative to wizards. Bottom line, they can't cast spells and ARE subject to magic and wizarding law. (Moaning Myrtle for example, is basically incarcerated in Hogwarts because she stalked someone.)
Do disregard anything Peeves can do, because he's a poltergeist, and doesn't follow the rules the other ghosts have (such as an inability to touch or move objects), but it is important to note that they have very little influence on the physical, and they never cast magic in their new form, which I think is the most important thing to remember.
Reasons why Peter would not.
- If he dies, he gets out of a bad situation, even if he doesn't know what's next.
- If Harry wins, his ghost may be punished by the ministry. If Voldie does...eternity is a long time to be tortured when you cannot die.
- Ghosts don't have power. Whatever you might think of Pettigrew, he was a brilliant wizard, even if it didn't show--the guy became an animagus. Even if he DID have help from his friends, it actually takes some personal power in order to do that. Also, he managed to cover up his own death and fool investigators.