Does bring up an interesting question. If the pitcher was an outstanding hitter would you ever consider using them as DH on their non-pitching days?
For the most part, only if the GM has done a poor job assembling his bench. If a team doesn't have a reserve hitter available who can't outhit all the team's pitchers, the GM has failed to do that. There have been, and are pitchers, who managers have used as pinch hitters. They are usually of two types. The first are pitchers who were awesome hitters in college and were scouted as such. Those would be guys like Brooks Kieschnick (who was an outfielder in the majors as well), Micah Owings, and Darren Dreifort. The other type are just the excellent athletes who never lost their hitting skills, such as Don Robinson, Mike Hampton, Victor Zambrano, and Livan Hernandes. Those guys pinch hit as well, and Robinson was decent at it.
However, a pitcher would have to be a very good hitter, to merit 4-5 at-bats over all of the hitters on a team's bench. While there are some potent hitting pitchers out there today, like Bumgarner, none of them merit a Dh slot.