GPS is reliable, but not perfectly so. Furthermore, it requires electrical power. A sailor who absolutely depends on electrical power to function is sort of missing the point, in my opinion. While the government side of the system has been remarkably reliable so far, there are reports of unexplained unscheduled system wide failures. Software can fail.
Coffee can spill.
A visitor might bump the big red switch. Unless a sailor to Hawaii is comfortable with dead reckoning from California, perhaps a sextant and almanac are a good backup. The Transpac is respecting tradition while encouraging a well rounded skill set. The problem with GPS and yachts is that it makes possible for people to go to sea, most of the time perfectly safely, without a sound basic knowledge of navigation. One can push a button, and presto-position.