The Dent Act reads.. "One of the main features of the Act is the creation of stricter penalties, including the denial of parole, for those who commit any crime deemed as a part or function of a larger criminal enterprise. This will close the loophole that existed in the previous laws that hampered the city's ability to stamp out organized crime." It's not just those who are already in jail. Anybody who gets picked up post-tdk who are attached to a bigger criminal enterprise are denied parole. In the beginning of TDKR Gordon is talked about as a war-hero. It seems like he was responsible for making the city pretty clean through the years. Garcia even says at "Harvey Dent Day" that you cant just wipe out ALL crime, but the Act is a massive help. A huge help when it comes to the big league stuff in the city. And that's what Nolans version of Batman was focused on. Not the random kids who are robbing folks from time to time. Or the odd murder in the Narrows. Perhaps Batman tended to some of that early in his career, but Nolan made a point of only showing Batman going after the mob and the big time freaks who put the city in jeopardy. Bruce targeted the mob mainly because that's who made the city what it was. It's what lead to Joe Chill killing his parents. With the mafia wiped clean, the chances of more Chill's committing those acts get reduced. The rogues were tied up in that whole mess, some of them were created by Batmans appearance. But Bruce was always after the big stuff. Batman was not needed in those 8 years because the Dent Act got whatever was left of the mob OFF the streets. It's pretty simple to me why Batman wasn't needed.