As argued by the aforementioned source, the use of KERS increases the centre of gravity within the Renault R29 by no less than 4 centimeters. This clearly affects the car's behavior on track, especially when referring to cornering stability.
Renault have so far confirmed that they won't be using KERS for the upcoming 2 races in Europe, in Catalonia and Monaco. While interfering with the R29's new aerodynamic updates, the technology would have proven useless on the twisty, long-straight lacking circuit on the streets of Monaco.
As revealed by the same Spanish publication, Fernando Alonso has consistently asked his team to use KERS only at high-speed circuits like Spa-Francorchamps and Monza. Hosting the Belgian and Italian Grands Prix respectively, the two venues are scheduled for late August and early September.
As far as the Spanish Grand Prix goes this weekend, only BMW have announced they'll let go of their KERS technology for both their F1.09s. Ferrari and McLaren, the other teams who have a functional unit to power their cars with, have confirmed they will use it in Spain.