It's 'Let's look at some numbers' time again.
Turn to page 191 of your shiny new 2009 Hardball Times Baseball Annual (don't have one? Matthew is displeased with you). Here Tom Tango discusses the impact of catcher defence by considering stolen bases, caught stealing, pickoffs, balks, wild pitches, and passed balls, controlling each for the pitcher in order to eliminate their effect on the battery's defence.
I'll summaraise the findings for you, in case you're currently invoking Matthew's wrath:
The worst defensive catcher of all time is Mike Piazza. The best was Ivan Rodriguez. Hardly surprising, huh? The really interesting thing about Tango's article is that it shows how many runs a year Piazza cost his team. He rated at roughly -1 win per season with his 'defence' at catcher. Pudge is worth about the same in the other direction.
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.
Now let's take a look at some positional adjustments.
C: +12.5 runs
1B: -12.5 runs
DH: -17.5 runs
Remember that the worst defensive catcher of all time was worth -10 runs. Apply this knowledge to Jeff Clement. With some work he could probably become a passably bad first baseman. He's never played it before and he's not particularly agile, but it's an easy position and he's a hard worker. He'd probably end up as a -5 guy overall. And unless you believe that Clement is the worst defensive catcher in history, that's only a 0.5 win improvement.
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.
Clement's bat can only take him so far. For him to live up to his potential value, he absolutely has to be kept as a catcher. Moving him from behind the plate should be the last resort.