Why did they have to choose a different person each time they went one level deeper in Inception?
Couldn't they have kept going into Yusuf's dream all the way down?
I think it comes down to one point - can one mind actively dream two or more states simultaneously.
For the dream within a dream thing to work .... the dreamer in level 1 (Yusuf) has to dream the Pasiv (sleep) machine into existence, in order to allow people to go down to the next level (the hotel dream of Arthur).
For someone to simultaneously be maintaining level 1 and be able to go to 'sleep' and maintain level 2 seems unlikely - you'd essentially have to be 'awake' in each dream state.
So it appears for the dream within a dream thing to work, you need as many people to dream as levels you wish to descend. This appears to be why Cobb wants Yusuf to 'go into the field', as he needs 3 dream levels (Yusuf, Arthur, Eames) - and he does not want to be a dreamer himself (for obvious reasons), and Ariadne was a late addition to the team.