Although Robin seems to have an pivotal role in ruining earlier Batman movies, he's still an integral part of the Batman series.
Is there a reason why Robin does not appear in Batman Begins/The Dark Knight/Dark Knight Rises?
It's a matter of the director having a specific vision of how his Batman stories will become. In the first two films, Christopher Nolan wanted to bring Batman back to his roots, explore how Bruce Wayne became Batman, and how Batman became the legend he is (in the 1989 Batman film, when Vicki Vale asks Bruce why bats, he replies "They're great survivors"; in Batman Forever, we get a VERY brief glimpse of bats on Bruce's mind. That's all the motivation we get). Batman Begins was very much an origin story, showing also the training Bruce took to become Batman. The Dark Knight follows soon after that story, as the ending of Batman Begins sets up The Joker to enter the story.
The thing is, I tried looking up the reason, and I can't yet find any quotes online from Christopher Nolan (see this article here for an expanded story on this). The next film, The Dark Knight Rises, supposedly takes place 8 years after The Dark Knight. It also features Bane, a character who became notorious for eventually breaking Batman's back. It's entirely conceivable that Nolan will include Robin in this film as his protege who can help him fight Bane. Nobody outside the film set really knows. Me, I'm looking forward to finding out in 2012.
Robin is in the final Nolan directed Batman movie ("The Dark Knight Rises"). In the final few minutes of the movie, when Officer Blake (Joseph Gordon-Levitt) is collecting his property from the boys home, the woman behind the desk isn't able to find his name in their system. He then gives her is 'real name,' after which she says "You should use your real name, I like it better (or something to that nature) . . . Robin."
The interesting part about the conclusion of the Dark Knight Rises is that it seems to be a set up for a "Robin" franchise in the next few years. When Officer Blake discovers the batcave beneath Wayne Manor (Now the new home for Gotham's Orphans) it seems to be a 'passing of the torch' moment. Also when he is talking with Bruce Wayne about the similarities of their pasts, and at the end of the movie when he is talking with Gordon about why he is leaving the police force (i.e. the 'structure'). In fact, during the first thirty minutes of the movie, there even seems to be a set-up for who the villain will be for Robin . . . Killer-Croc. When Commissioner Gordon is taken captive by Bane in the sewers, there is a brief mention by one of the officers of a "large crocodile" (or something using those terms) in the sewers. I believe that Killer-Croc spent the majority of his time in the sewers in the comic series. It seemed like an off the cuff kind of statement for a character, and pointless to have in the movie, since it's humor factor didn't really have too much of an impact. I honestly believe it was a direct set up line for a Robin movie.
To keep it short:
The point of the Robin reference was to display that even though the audience did not know Blake was Robin, he still was. In some interview, someone close to the production of the movie mentioned how the purpose is that he was Robin the whole time, you only find out at the end. This represents perfectly what Batman says to Blake earlier in DKR, that his goal was only to inspire people. The purpose of Batman is not that only Batman can do it.
The message is that anyone has the ability to stand up for justice and protect his or her society. Some of the true heroes of the day are Gordon, Blake, and Selina (Selina actually SAVES Batman when Bane is about to kill him at the end of the movie). Making a Robin movie would veer away from the purpose and just present another hero of Gotham, while, conversely, the hero of Gotham is the soul of hope. And that belongs to anyone.
Nolan has given an explanation for this. According to him the Batman portrayed in his movies is still young and according to comics Robin appears only when Batman grows old and hence needs a sidekick. Nolan has also said that in his trilogy Robin won't appear at all because these movies will present a young Batman.
Christian Bale said that he wouldn't want to be Batman in this franchise if there was a Robin and Christopher Nolan obliged. Nolan said that he wouldn't fit the dark tone of the movies. Also Dick Grayson is very young when he becomes Robin, about 10 or 12. So that's another reason for not having him in the films.
Batman will be rebooted to fit in with the new DC Universe leading up to an eventual Justice League movie that has been green lighted after the success of Marvel's Avengers. It will not be confirmed until after the Superman reboot that's coming next year. So there was no time to introduce Robin as the Franchise had to go to make way for what they expect to be a bigger movie, believe it or not considering the success of Nolan's Batman franchise.
That being said, and after watching the ending of the TDKR... they're obviously toying with the idea of a Robin or Nightwing franchise in the Nolan "realistic" Universe because it is such a wildly popular adaptation.
I expect to see a spin-off Franchise dealing with how Robin becomes Nightwing in a few years. Otherwise what was the point of that ending?
Nolan spoke to this in an interview for Empire-Online. In short, he simply found it difficult to imagine Robin fitting into the world he'd created:
Q. We have to ask about the Robin gag at the end. We’d always wondered how possible it would be to make that character work in this version of the Batman universe, and this has to have been the only way.
Nolan: It is a little hard to imagine Robin working in that universe, so the idea had to be limited to that gag at the end. But Joe’s character is very important to the story. In any movie you need a character looking at proceedings the way you see them, and Joe’s character is that character for this film. One of my favourite scenes is when John tells Bruce how he knew he was Batman. It’s like that scene in The Prestige where the little kid sees through Christian’s trick. Little kids, they don’t have any illusions, they just see the truth of the situation. I feel there’s a kind of spiritual connection between the two movies there.