Although Nick hasn't been setting the track alight recently, he still sits two points in front of Petrov in the drivers' championship. Heidfeld is remarkably consistent, which Petrov and Senna lack. Petrov has been making strides in this department this year. Senna has yet to race this year.
Renault are set to release a major update package this weekend, which Heidfeld certainly had a hand in developing. I don't want to overstate the driver's impact in car design because I feel it's negligible from an engineering standpoint. However, the driver's feedback helps steer development in the direction of their preferences. This update will be based on Heidfeld and Petrov's style, not Senna's. Why are Renault choosing this race to give Senna a shot? They should be seeing how the update pans out first, and then bringing in Senna if they want to give him a shot.
Eric Boullier had been expressing his dissatisfaction with Heidfeld for a while previous to this announcement, so it's hardly a surprise. However, Heidfeld was never going to be a complete stand in for Kubica. Kubica is a world-class driver who can't be replaced. Heidfeld offers stability and experience, and he has delivered in these two areas.
If the decision were taken to pick up some Brazilian sponsorship money, fair enough. Teams need this money to perform. If it were taken because Renault honestly believe Senna will outperform Heidfeld, I think they're sorely mistaken.