To be fair, that's not exactly what happened. After Bruce saves the Robins, Duke shows back up with Gordon to try and help, but they get captured by Bane's henchmen, only to be saved by Selina right afterwards. But I don't think it necessarily means he had any concrete important plans for him when his run really got going. There very well could have been major plans editorial wise for him, but like you said things got changed or scrapped, and King used him sparingly enough that it didn't feel super jarring that he didn't show up that much later on if he was initially important.