Ra's and Chill didn't leave a lasting impact on Gotham. Gotham didn't know Ra's or the LOS existed. Everyone in Gotham knew and feared the Joker, even the underworld, who would rather get their ankles broken by Batman than cross Joker. Joker affected every area of Gotham. He was killing Cops, judges, he mutilated their beloved district attorney, blew up a hospital, took dozens of citizens hostage, caused a city wide evacuation, murdered Batman's best friend a love of his life, took over the underworld while killing several prominent mob bosses and burned half of their money etc. That's just the antics the people of Gotham know about. There's the whole mess with what he did in breaking Dent and leaving Batman to carry the can for Dent's actions as Two Face.
What on earth makes you think bringing up the Joker will have no relevance to something happening in the movie? For example can you not envision a scene where the whole business of Batman being a hunted criminal for several murders is being dealt with, and Batman mentions he had to take the fall for Dent because Joker would have succeeded in breaking Gotham? Can you not see him discussing the consequences of this with Gordon? Or Alfred? Maybe even confiding in Selina when she asks is he really a killer?
Then there's Bane. He obviously finds out about what happened to Dent if those pics of him standing in public and tearing up photos of Dent is anything to go by. How does he find out about what happened with Dent?
What makes you think it would be pointless? The legacy of the Joker's actions is going to live on in TDKR, or have you forgotten he's the reason why Batman had to take the blame for what Dent did? Can you really imagine talking about what happened to Dent and not mention the Joker, since he was the cause of it all?
Yeah, that's exactly the scenarios we are all imagining it happening in