If I had to guess, I would say the inclusion of Tennant and Rose are specifically to help remove Clara (who is signed on for season 8) from the Doctor's time stream and back in the time line. Simple enough, following that logic you could remove 11, as well, and send them on their way, right? Wrong. Seeing as 11 entered his own time stream (something a time lord must never do) I'm guessing there's a fair bit of nastiness that completely prevents him from leaving his own timeline. Plus, let's not forget that he is regeneration number 12. If you don't hold onto the theories that his regenerations from River passed on, then he is in all respects dead to rights after this death.
So what if 11 wasn't The Doctor who regenerates?! He would forever be trapped within the timeline and able to appear at any moment to save the day (much like Clara). Tennant of course is now half human so he wouldn't be the one to change, but of course who better than "The Doctor Who Broke His Promise" aka true Number 9 with three regenerations on standby to take on the continuation. There are some parts of The Doctor's memories that I haven't quite worked out yet and how that would work...but I'm sure the Tardis holds a memory bank of the Doctor's thoughts so he could be brought up to speed. Ultimately it would ensure Smith would have the option to return if he wanted. I dunno. Is my theory too crazy?