You know you've written yourself into a corner when you have to create a whole new character just to resolve the plot. The Meta-crisis Doctor was basically a living fusion of deus ex machina and fan service. First, he provided the means for Donna to escape the Dalek Crucible and gain the information required to swoop out of nowhere and save the universe. Second, he created a way for Rose and the Doctor to (sort of) live happily ever after. With his job done, the real Doctor quite happily abandoned him, together with the woman he supposedly loved, on a parallel world and closed the door behind him forever. Or, at least until they need a role of David Tenant to play in a future reunion episode. Ironically, not only is this one character that we all secretly hated, but he was the character that would have been easiest to kill off. He couldn't regenerate because he was half human so it would have taken just one shot from any of the thousands of Daleks swarming around to put him back in the ground where he belonged. And speaking of thousands of swarming Daleks...