Well, I don't know for sure that the Phillies are a poor fit for A.J. Burnett. He'll only be two hours from his home in Maryland, and maybe family time is just that important. If going east works out better for Burnett personally, then hey, good for him. He'll get more money than the Pirates would have given him, too.
As a baseball move, though, this is bad, mostly for reasons Keith Law recently outlined (Insider-only). The Phillies finished third-to-last in defensive efficiency last offseason. Chase Utley and Jimmy Rollins still form a decent defensive middle infield, but first baseman Ryan Howard is well below average, and likely third baseman Cody Asche appears to be as well. Burnett, meanwhile, has had a ground ball percentage above 56% in both of the past two seasons. The Phillies are going to have trouble getting to all those grounders.
We know Burnett didn't like the Pirates' shifts. If he wanted the Pirates to shift less, he's likely going to wish he'd been careful what he wished for, because, at least through mid-July, the Phillies had shifted less than any team in baseball.
Then, of course, there's the fact that the Phillies don't look like a contender even with Burnett. Is it sour grapes to point out that the Phillies don't make much sense for Burnett? Sure. But this is truly a situation where no one really wins. The Pirates lose a very good pitcher who was well-suited to their defensive personnel and strategy (whether or not Burnett himself believed that to be the case), the Phillies get a good pitcher who's poorly suited for them, and it's likely that Burnett and both teams are going to pay the price. Oh well!