Because it needs to be repeated and understood...
Journey's End bothers a lot of people in several different ways. There's Donna's ending. There's the Doctor yet again taking freedom of choice away from his companions. There's Jack and Rose never getting their closure, and Rose never finding out what she did to Jack (part of my wish-list for the episode). And there's the small matter of two Doctors and Rose being made a present of one of them to keep.
Donna's ending, frankly, infuriates me - not for what the Doctor did, but for what the writers did. In-episode, I don't think that the Doctor had a lot of choice given that she was dying right in front of him. But I do think that the character should have been given a more dignified exit.
I'm happy for Rose. I think she'll do well with her human Doctor. I wish the alien Doctor had, for once, given her a choice, explained the reasons why he felt it was best for all of them if she took his other self, the half-human part of him, back to the parallel world, and let her decide her fate freely. He's taken her choice away too many times, and it's demeaning. At least half-human Doctor was giving her a choice: "I've only got one life, Rose Tyler. I could spend it with you. If you want."
However, the one thing I'm not remotely upset about is the existence of the half-human Doctor, or that Rose got 'stuck' with him. What does irritate me, though, is seeing comments and rants along the lines of 'he's not the real Doctor' or referring to him as a clone or a copy or even an abomination. Suggestions that he only has implanted memories because he's only been alive a few hours. I understand that it's partly resistance to the idea that Rose doesn't get to stay travelling in time and space with the Doctor we see on our screens, but she was never going to get that as her ending. This way, though, she gets to spend her forever with the Doctor. Rose/Doctor is canon. Shippers are never going to get better than that.
The half-human Doctor is the Doctor, as he says himself to Donna during the episode. He is a regeneration, not a copy or a clone. And if you want to see it argued out in full, with science-geekery and Classic Who meta, you won't see it put better than dark_aegis does here.
If you don't like the character of the half-human Doctor, fine; but please, stop calling him something he's not. And that's all.