I think it was more that if Dent's image was "tarnished" a number of things would happen including him being dis-barred (thus meaning he couldn't prosecute for the RICO case), someone else would step-up as DA and that person may be better influenced by the crime in the city to get the RICO case lost/thrown out.
They needed Dent because he was a clean, good, lawyer who wanted to solve the city's problems and they may not get that with another DA. His image being tarnished by his involvement in crime would really mess things up in prosecuting the criminals.
So I saw it as being less about his image and just more about the politics and practicality of the whole legal system.
Well, one of the major issues of suspension of disbelief in viewing the movie is buying the whole "DA manages to prosecute hundreds of mobsters in one case" conceit - that's pretty far-fetched for a police procedural story, which is the style The Dark Knight mimics, though it's not at all out there for a superhero story.
If you somehow believe that society is an illusion held together by mutally agreed upon symbolism, I suppose the hysterics about Dent's image wouldn't seem so absurd. I don't understand how anyone can be so confused about the world, though. It's like thinking that the Joker's nihilism is a real philosophy or a real mental illness or a real anything.
The basic premise of the Nolan Batman movies is that the symbolism of powerful people is critical in the life of Gotham City - it is the hinge upon which Bruce's entire conception of Batman turns in Batman Begins. Carrying that forward into the story told in the Dark Knight is extremely thematically consistent. However, if you can't buy that premise then the movies aren't going to work for you at all.