I'm not sure it's ever explicitly explained why Schultz brings Django to Candieland in the first place?
Surely they were aware that their plan is greatly jeopardized by bringing Django along: not only does he have to temper his emotions around Broomhilda, they are also counting on Broomhilda being able to do the same with no prior warning.
Django has already fulfilled his purpose in proving to Candie that Schultz is interested in Mandigo fighting at the Cleopatra Club, which is how Schultz obtained his entry to Candieland. This proves Django was no longer needed to convince Candie.
Schultz was already very capable of handling himself, being a bounty hunter.
And as for 'back-up', surely the last possible thing the pair wanted was a Gunfight, so why anticipate one? It only happens due to the failure of their original plan.
And as for being able to positively identify Broomhilda, the dialogue already mentions that the chances of there being two African slaves named 'Broomhilda' are almost zero.
As far as I can see, the only thing that would jeopardize the plan is Django's presence, so why does he come along?
Edit: No 'because it makes the plot better' answers, please ;P. I know its a great movie, but I'm looking to see if this is explained within the movie.