I posted this exactl same fanpost on Brew Crew Ball. It relates mostly to the Brewer point of view, but does touch on a few things Giant related. Apparently, this trade was out there all offseason long and Sabean did not want Fielder on the team for defensive reasons. I post it here as well to get the other side's opinion.
Many posts and stories have been done about the Brewer pitching staff, and its recent decline. Like many of the frequent visitors to this site, I am of the opinion that the Brewers need to add a starting pitcher to make a playoff run this season. Manny Parra's slide has only exacerbated the weakness that the Brewer starting rotation is not quite built for to make a playoff run.
But what to do? I have always thought that the Giants would be a perfect place for Fielder after his time was done with the Brewers because of salary issues (the Giants or a big market AL club where he could be a DH). The big question is whether or not the Brewers could score enough runs to win without Fielder in the lineup. There is no doubt that the team needs another pitcher. The nice thing about dealing for a guy like Cain would be that he is currently cheaper than Fielder and he would be under team control for two more years.
Fielder currently has a 2.5 WAR vs. Cain's 1.5. That could be balanced by the Giants sending Travis Ishikawa over as well to balance out the trade. Ishikawa is currently at 0.5, but the difference between Fielder and Cain's salaries would at the very least make up for that 0.5 WAR lacking on the Brewer end. If anything, the Giants might even want a prospect going their way to make it a deal worth considering.
In a trade like the one I mention above, the Brewers get an upgrade in the pitching staff, they get a replacement level player to take over at 1B, and in the end improve the team overall. The Giants have a ready made replacement for Cain in either Tim Alderson or Madison Bumgarner (yes, they are both MLB ready now) and they upgrade their offense to make a possible playoff run.
One other possibility would be Cain for JJ Hardy and Corey Hart. The problem is that you replace Hart with Gerut, which in the end may not be all that bad, but Gerut has not performed well at all since joining the team. I have no fears at all about Alcides Escobar taking over for Hardy at SS though, as his defense would improve the pitching overall, and his offense couldnt be much worse than what Hardy is giving the team right now. The giants would improve themselves offensively here as well, but Hardy might have to play 2B until the Renteria contract expires and no body knows if he could actually do this.
Will Sabean pull the trigger on a deal like that (either of them listed above), probably not. But would you?