The more I think (and read) about this trade, the more I like it for a few reasons. The first reason should be obvious; Iwamura makes the Pirates better at a position of need without giving much up. He's not blocking anyone from playing in the immediate future, so why is becoming better in 2010 a bad thing? The Pirates are mostly done tearing things down; at some point they have to begin building up. Iwamura gives the Pirates a lot of flexibility in an area they previously had none. If he plays well and is open to an extension, Andy LaRoche becomes expendable if he can't adjust to second base. If he plays well and the team is bad, they'll likely get more than Jesse Chavez in return for him. If he plays poorly, isn't traded, and doesn't re-sign, the Pirates haven't given much up.
The other reason that this is a good move for the Pirates is that it gives the Pirates their first significant Japanese player. Michael Street has a good post on this at Baseball Digest Daily. Beyond vastly expanding the Latin American operation since 2007, Neal Huntington's staff has signed players from South Africa, Taiwan, India, and Australia. Trading for Iwamura furthers the movement to rebrand the franchise as an international one to potential international signees.
This isn't a move that turns a 99-loss team into a playoff team or even a .500 team, but it's hard to argue with bringing a useful player like Iwamura in without giving up much. There's just not a whole lot of downside to this move, no matter what angle I look at this from.
UPDATE: While I'm tossing links about this trade out, Dave Cameron has a very good analysis of what wins mean to a team like the Pirates at FanGraphs.