The Mets and the Cardinals played 18 scoreless innings last night, with the Cards ultimately losing 2-1 in 20.  The losing pitcher was St. Louis centerfielder Joe Mather, who also played 3rd base after entering the game as a pinch-hitter. 

Non-pitchers have been particularly bad as pitchers over the last 40 years:

Ouch!  That's basically the pitching equivalent of a pitcher hitting.  You are certainly better off letting a tired reliever continue to pitch (provided you don't think he'll hurt himself) than putting Jose Canseco on the mound (and having him hurt himself anyways.)