This is really bugging me, and it all goes back to Graham's post a few weeks ago (thanks, Graham!):
So the difference between the worst catcher of all time and the best is on the order of 2 wins a season. Which isn't very much.
For 5 runs of defensive upgrade, Jeff Clement would lose 25 runs of positional value. 25! Potential injuries aside, no matter how bad he looks in the field, there is no way that you shift Clement off his postion right now. As an average major league hitter, a level well within his ability to reach, he'd be worth 2-2.5 WAR as a catcher against 0-0.5 WAR as a DH/1B. Start being less conservative with his defence (after all, he's not going to be Piazza bad), and the difference only grows.
The implication here is that if a player is physically capable of squatting and catching most pitches, he should be used as a catcher until he is no longer capable of doing so.
Just for fun, let's try this with, I don't know, Raul Ibanez. You could substitute Adam Dunn or Pat Burrell or Bobby Abreu any of the other "should be DHing" types if you like, but Ibanez at least has worn a catcher's mitt at one time, so he's the most 'realistic' if you can even call it that.
Let's say that Ibanez is top-tier, first ballot, Poopertown Hall of Shame worthy. Half again as bad as Piazza as Piazza was as bad as average (-1 defensively). Theoretically, this only costs the team 15 runs? He gets 12.5 back just as a positional adjustment, which makes it almost a wash defensively.
Fangraphs has Ibanez at 2.3 WAR with the bat, but just 0.55 overall as a LF:
At -2.5 runs as a defensive catcher (-15 for being historically laughable, +12.5 for the positional adjustment), he's a +2 win player, and totally deserves his contract, for the first year, anyway.
If Piazza was -10 runs defensively, Ibanez would have to be about -30 to justify making him a LF instead of a catcher?
I do my best to follow all the stats, but I must have gotten a few crossed up here, because barely-replacement-level LF just don't go on to be league-average catchers, right?