It reduces stress on the drivetrain, a small turbo spools up instantly and can produce a shockload on the tranny/diff/axles. Simply slowing down the quick spooling turbo alleviates this.
Did you notice in the ic pipe dyno thread that the tps only opens up 83%? Pretty stupid imo, but from gm's standpoint it makes sense to limit output from the engine. Im sure theres more nanny controls we'll run into, these seem to be the first 2 that were found.