I recall reading somewhere that they stopped using blown diffusers a bit before 08 because they really upset the balance and stability under the current aero regs.
Can't remember where I read it though
Scarbs had a good blog about it. Ferrari were the first to move away from the blown diffuser in 98, exiting the exhausts over the engine cover. I think the move happened for a number of reasons. One is, as you say, the sensitivty to throttle positions. The second is that heat resistant materials were not as advanced back then, and the exhaust gases sometimes ended up deforming the rear suspension. Final reason is that the 98 regulations meant the engines had to move further forward, relative to the rest of the car. This meant, in order to blow the diffuser you needed longer exhaust pipes, which was not good for the engine (i think the engine works better at higher revs when the exhaust pipes are shorter - this article explains it in detail http://www.grandprix.com/ft/ft00360.html
It is interesting that Newey was the last designer to keep the blow diffuser, and it seems he grudgingly got rid of it at the request of Mario Illien. He was also the first to bring it back. He's never really produced a dominant car without it. He definitely seems to know how to exploit it.