On top of what's been said, a good reliever is better than a situational match-up.
The Angles have had the best bullpen ERA in baseball in each of the past three seasons, and don't bother much with lefties. K-Rod is a perfect example. Lefties have hit him 63 points higher in his career, but if they're still only batting .202, who really cares? The Angles pen has been great this season as well. They've allowed four earned runs in their last 39.2 innings and have stranded 22-of-23 runners this season. They've got only one lefty in their pen, a rookie that has thrown a whopping 8.1 innings.
The overrating of the lefthanded specialist allows guys like Jesse Orosco and Aaron Flutz to stay in major league uniforms for seaons longer than they should.
I honestly can't see the Pirates trading Gonzalez though, as its already been said, he's very, very good, and very, very cheap right now.