@NANLIT:
Why would there be a defend AL arc? The world is in chaos due to the influx of pirates going to the sea because of Newgate's death so the WG has their hands full to worry about any problems they may have against Hancock (not t o mention Teach and his crew). And as stated by Garp, the WG has no idea where Luffy is, so that can't be a reason to attack AL.
Well, as things stand now you're right. However, even without knowing Luffy is there, the marines could realize Hancocks actions either through her attacking Smoker, or the Pacifistas, or whatever. I don't really think it's likely, but if Oda did it I'm sure he would explain it well, so I wouldn't really be against it. However, I fully expect all these new power ups and such to be displayed either in, or passing into, the new world. That just seems to make the most sense to me, but you never know with Oda.
EDIT: You also raise a good point clover, after defeat it's unlikely that the Marines would just give up, although that could play back to Nanlit's point. Maybe they would be too busy to worry about continuously attacking. As I said though, I don't really expect it to happen, but if it were to, I'm sure Oda wouldn't leave plot holes.