Is someone with good route knowledge 100% going to be able to find that optimal line even when the visibility is down to a few meters as it was on Saturday? Probably not. Is the GPS watch going to be able to? As long as it has satellites, yes.
Agree here, also ripping a .gpx track off (which is why some of my more 'useful' GPS traces are now hidden).

In response to 'isn't following someone you know knows the route the same thing' - I refer to the point above, but also that following someone in a race scenario is an in-race tactical decision and part of racing - when to follow, when to take a slightly different line, when to hold back and when to push on.
Deciding that you will listen to your watch beeping every time you go off-line, whilst in some ways a tactical decision, is pre-emptive and not a racing decision.