So, what's the Biological issue?
If I remember right in in the original series they never gave a reason why. They just stated that they couldn't regenerate more than 12 times. At a guess I would figure it's because their body's cells can only handle being rebuilt 12 times.
Maybe that's why Moffat decided to create an extra Doctor, maybe, because he wants to address the 12 Regeneration rule, and be the one who gives the Doctor new lives before Moffat moves on to his next job?
Yup. He wants to be the one to extend or undo the regen limit.
Everyone always forgets about poor old Richard Hurndall.
Not to mention Michael Jayston and Toby Jones.
It's the fact that Capaldi is the 12th actor to play the role - this is officially designated by Moffat and co during the press release - and yet John Hurt is apparently a 13th actor. So he'll probably get swallowed up by a crack in time or the Doctor is reset/gifted another regeneration - but still doesn't hide the fact that - if John Hurt really is "the Doctor" between 8 and 9 - Capaldi should be 'officially' the 13th, because biologically speaking he has to be.
I agree with everything Candlelight has said. The Doctor in his Ninth and Tenth form has been pretty open about he did, and never gave an inclination that he had something to hide in terms of what he did do, which was to end the Time War by wiping out all Daleks and Time Lords. More importantly, WE know this, as an audience, so that doesn't really leave much in the way of mystery.
And even more importantly, I'm not sure the Doctor in his 11th incarnation hid it away, either - Emily seemed to know pretty well of what had happened to the Time Lords, and he frequently mentioned he was the last of his species.
So, really, if the shock twist of the Doctor in his Hurt form has to do with the actual termination of the Time War... Russel T. Davies beat him to it.
Going by the dialog at the end of "The Name of the Doctor" Moffat is setting up that the Doctor has no problem admitting what he did. He's just so ashamed by the incarnation that did it that he disavows his exsistance.
Basically, it's cheap and easy way to add another incarnation and keep the current Doctor numbering order the same by having selves 10-12 (possibly 13 as well) strip the 9th of the Doctor title.
