Saito wanted to be present to ensure that the job went his way and that the team was successful. Having already experienced a failed attempt at information theft (when Cobb and co tried to steal information from him), Saito wanted to put every little thing into his favor, and the best way to do that was to be present at every step.
A good example of this is during the planning phase:
On a 747 the pilots are up above, first class is in the nose so nobody
walks through the cabin. We'd have to buyout the whole cabin, and the
first class flight attendant-
We bought the airline.
Everyone turns to Saito.
It seemed... neater.
Neater, huh? (gets to his feet) Well, now we have ten uninterrupted
Also, Saito is the key for Cobb to enter the US without getting arrested. When Cobb's wife killed herself, she thought they were still in the dream, and she wanted Cobb to join her. As such she set it up so that it looked like Cobb killed her, hoping that he'd kill himself to follow her. He is still a person of interest in his wife's death:
If I get on this plane and you haven't taken care of things...
when we land I go to jail for the rest of my life.
Complete the job en route, I make one phone call from the
plane... you will have no trouble clearing immigration.
If Saito wasn't on the plane when the plan completed, there's a chance he might not have had enough time for Cobb to call him, tell him the job is done, and then for Saito to call his contacts. Again, being in person was the most sure way to hold up his end of the bargain.