So, I was wrong.
If you go to Werth's stance in the editor, it is indeed listed as "Generic #1" and it shows his stance as the generic stance.
What I had forgotten, though, is that MLB The Show has 'hidden' stances and pitching motions in the game, and although it says "Generic #1" for Werth, when you play the game his stance replaces the generic one instead.
So, word for all those worried about Werth's stance in MLB The Show 11... don't edit it, it works fine when you play with him, in game.