In that scene, even if he did take Watson's help - I don't think injured Holmes + Watson would beat Moriarty.
Leaving aside the possibility of Holmes planning to fake his own death, I have an alternate and more Sherlock movie like theory as of why this happened:
I say that Holmes didn't expect Watson to come at the end of their fight and quite possibly - instead had the "area" under the hill (that valley) and the dynamics of his and Moriarty fall figured out (which Moriarty definitely wouldn't have thought about - according to Holmes) just before he made the jump. He also planned the jump in such a way that he would somehow escape the fall (quite possibly by falling into the water, while Moriarty falls on Rocks).
We can see Moriarty screaming all the way down, but Holmes keeping his cool while falling, which mildly indicates this.
Also, by the time Watson comes in, Holmes caught Moriarty in such a way that in a split second Moriarty knew what Holmes was going to do. Given a little more time, Moriarty would have managed to push Holmes down (because he is towards the wall) - So, Holmes didn't have enough time to change his plan - otherwise Moriarty would kill him, and most likely Watson too after that. This is why, he continued with what he planned before.
So, I guess through this scene - Sherlock is shown as more Intuitive and "outside the box" sort of planner when it comes to his "fighting simulations" compared to Moriarty. Clearly, Moriarty didn't expect the possibility of both of them falling down and Sherlock wins!!