I'm reading a lot of "well Dunn's actually fielding ok this year..." type of comments to justify signing him for additional years. While technically correct, these are probably misguided.
Just because Dunn is having an average year in the field does not mean he is an average fielder.
The same UZR that has him as -1.9 this year (just barely below average) had him at -14.3 last year (terrible). This isn't that surprising. Current fielding statistics are notoriously variable from year to year. You are meant to look at them across a couple years or so to get a good idea of how well a player fields. Dunn's been pretty terrible at first, in admittedly limited play, since 2005. This agrees with the general opinion of most scouts.
You can take this in one of two ways. Either you can believe that this year is a fluke, the fielding equivalent of a Pudge's .400 start to 2010. In that case you'd expect next year Adam will once again struggle in the field. On the other hand, you can believe this is the end result of an effort to make himself better at a much easier position to hide his faults.
If you believe the former, then the move to a Derek Lee or Carlos Pena makes a good deal of sense. Even if, like me, you don't think it should happen. (If in the end it gets you about the same amount of "wins" - I'd rather have sucky fielding and 40 bombs for 12 million a year, than good/ok fielding and someone hitting .200 or having only 15 homers for something like 5 million. I consider it a 7 million dollar investment in fun. Smart spending to some is cheapness to others).
If you believe the latter, then it's true that it makes no sense to not resign Dunn. He'd be a top 5 first baseman in the NL. However, you have to understand you are taking the bigger gamble. You are betting on Dunn repeating a passable defensive performance something that he hasn't had previously since his rookie year in the OF.