I'm reading through the 2007 Hardball Times Annual (which I highly recommend) and I just went through the chapter on the outfielders with the best arms. After breaking down the best outfield arms of each era, John Walsh goes through the worst outfield arms since 1957. Let me just put it this way, the following players have worse arms than Juan Pierre:
Frank Howard
Don Baylor
Juan Pierre's arm costs a team 5.4 runs per season versus the average centerfielder, over a run more than Al Bumbry, the second saddest centerfield arm of all time. Since Juan Pierre's base running was worth a little over 15 runs from 2000-2005, or about three runs a year, his arm more than cancels out whatever gains we may get from his treasured legs. For even better news, a player's arm tends to decline over time, so Pierre's arm isn't even close to as noodlely as it will be by the time his contract runs out.
If you doubt the validity of this study, it pretty much matches up with our observations of who has the best arms. For example, these are the best right field arms of the last few years: Larry Walker, Raul Mondesi, Bobby Abreu, Vlad Guerro, Orlando Merced, Mark Whiten and Ichiro. Interestingly, most players' arms make very little difference. Brian Jordan has the 10th best arm of all right fielders in recent years, but he's only saved 9.5 runs over average in his career. A player needs to be notably good or bad for his arm to have any real effect on the game.
When I noted how weak the Dodger outfielders arms were I wondered exactly how much that would affect the team. If Gonzalez and Ethier's arms are anywhere near as bad as Pierre's, the answer could be as much as one win. While for the most part a player's arm doesn't matter, you should look to avoid the set of girly-armed outfielders the Dodgers have assembled.