Abreu opposed it last year. I think it's a bit of an arrogance thing. They put him there for a couple of weeks and his numbers tanked. This year, he's in the perfect spot. He's getting on base when no one's on in front of him and he's driving in runs when they need him to.
The other thing is, since Rollins is having a nice year, there's really no other spot in the lineup that fits him better than leadoff. If Abreu leads off, you'd have Rollins or Polanco/Utley hitting 2nd, Thome or Burrell hitting 3rd. That's too many strikeouts in the 3 hole. Bobby is a much more patient hitter than those two.