I don't think people who know what they are talking about claim that exclusives sell more. Most people I would assume know this. The reason people want such and such game to be exclusive is not sales of the software. They want the exclusivity to increase the sales of the hardware. Exclusivity also can be a benefit to the development costs even if they sell less they save money on porting or optimization. Release windows can also be more accurate and shorten total development time. In that same fashion, quality can also increase if a game is exclusive. Freeing up both time and money along with only needed to code for one device specifications allows for more optimization and less trade offs to establish parity.
Before the PS3 everyone was nice to me :(
