I'm not a fan either. But the way to justify it would be for Bruce to decide he wants to groom a successor, should anything happen to him, because that's how important Batman to Gotham.
If it's done-
Dick Grayson shouldn't be someone out on every patrol. Bruce calls him in infrequently, to give him experience.
And I would be interested to see how Nolan would re-design his look. No flimsy brights colours, but armoured protection, complete with helmet and mask/goggles I'll bet.
Maybe he would announce his presence differently, with a piercing bird shriek (recorded) to freak out bad guys, which is the opposite of Batman because he approaches silently. Just ideas off the top of my head.
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Right! Now, I'm on a mission!...If I can just remember...What?! I've got no time to talk - I'M ON A MISSION! And...what planet am I on again??
A third movie would be to soon for Robin. Bruce Wayne hasn't been batman for very long and it would also ruin what the Dark Knight set up. The third movie is going to have to deal with Batman being a hated fugitive.
__________________ "If you tell the truth, you never have to remember anything" -Twain
(sig by Scythe)