Accuracy is only one performance item to be respected by a navigation system. I write NAVIGATION system. GPS is by all means just a POSITIONING system. Only thru software programs using databases one can make a Navigation system out of the GPS. However, the other three factors are availabitliy, reliability and integrity. On the last parameter, i.e. integrity, GPS fails in more than one respect to be a precision system. (think of these databases being filled by humans!).
Nevertheless, for en-route purposes itīs a hell of a system. I love it too. Especially in my CAR!!
But to be a true precision NAV sytem, one has to consider definitely other system like MLS. But off course...as the yanks didnīt invent it, they donīt adopt it.