The rationale is that when the shooter has made the determination to fire, disengaging the safety needs to be accomplished in the quickest way possible to enable one to shoot. While some of have trained themselves to disengage the stock safety, others think that the stock safety is slower to disengage than a domed one. Quicker on the trigger is the belief. The domed safety is not, in my years at the range, less safe but instead an adaptation and refinement for those who find that it meets their needs. In the hands of those who are properly trained it isn't unsafe, much like extended safeties on a 1911 or even an ambi one. It's an implement to facilitate a resolution.