Spoken like a true gentleman Cancer4TheCure. You're probably right I am looking into things a bit too deeply but I just feel that this is such an important part in the movie as it is one of the reasons Batman takes the fall for Harvey.
I'm gonna' just say you may have missed the point as to why Batman took the fall for Harvey. Dent gave Gotham City something it hadn't had since Thomas Wayne was alive...hope for a better tomorrow. Harvey Dent "was the symbol [Batman] could never be." He wanted to preserve that symbol so Dent dies a martyr, a victim at the hands of the Joker in his crusade to make Gotham safer. Perhaps, like his parents' death, this will galvanize the city to take up his cause.
He didn't do it, because he wanted to make sure the massive trial Dent's office was pursuing would be completed. As you pointed out, with Lau dead there is no evidence and basically the entire thing falls through. No convictions or guilty pleas.
I'm pretty sure everyone figured Lau was dead when the Joker got him and with the DA Office in Gotham City essientially decimated (especially if you count the disappearance of Finch just 12-18 months before TDK takes place), traditional legal justice is not what Batman is sacrificing himself for. He is doing it to preserve the quality of hope in Gotham City.
Also, the arrests they were talking about when they saw Dent's body was all the major arrests he made earlier in the year. It is apparent he has made inroads on middle management mafia as Maroni was on trial at the beginning of the movie and that means some heads had to have rolled somewhere. Lau also mentions "You're enthusiastic new DA put all my friends out of business," and Dent coming from the IA office of GCPD apparently took down a lot of corrupt cops as soon as he got into office, hence the tension between Jim Gordon and Harvey Dent throughout the movie and the lack of trust between the two.
And then he must have done some other work as there were a crap load of convicts who didn't look like mobsters on that ferry at the end and they were "Harvey Dent's degenerates" that he prosecuted. In all honesty considering he was only in office for a year, he did an unbelievable amount of successful prosecuting.
But yeah, all those arrests would fall into question and the ones important to organized crime (if there is any organization left after TDK) would get out leading to more escapes because all the work of a homicidal/insane DA would come into question.
But again it is more about the bigger picture that Batman did what he did.