No, he's a running back who happens to catch the football better than any other back in the league. His talents out of the backfield don't negate his talents as a halfback. He's a terrific runner.
Duce didn't wear out his welcome in Philly, he just wanted a chance to be an every-down starter. He ended up being a complimentary back in Pittsburgh, just as he would have been in Philly. The Steelers may have been happy with him in that role, but that's not what they paid him feature back money to be. What they're happy with, more than anything, was how well the team did this season.
He was injured during the Pats game and missed several games. Bettis stepped up and the Steelers went with the hot hand. Seeing as how the Bus is going to the Hall of Fame no one can really say anything about giving him time.
Westbrook is a 3rd down back/spread out guy that needs to get touches. But, run him up into a tough defense and he won't last. I look at someone like Corey Dillon who looks to hurt people when he hits the pile and that's what I want in a running back. I'd take Westbrook on my team anyday and use him to screw with the other teams' defense. I'm just not going to rely on him to control the clock late in the game.