Not done the BG but from reading forums I gather that most of the good lines have nice elephant tracks on them nowadays! These lines show up beautifully on Google Earth and are very useful for plotting a race route. It's worth picking your favourite race and having a quick look at it on Earth; you'll be surprised how obvious the 'race route' is even if it doesn't follow an official footpath. A gps trace will get you to any summit you want, pretty exactly. I've never had a gps affected by the weather conditions although I accept that it's possible.
I'll freely admit to using a gps trace in a race. The first year I did the Wadsworth Trog I got badly lost between High Brown Knoll and Sheepstones; ended up cutting down to the road and abandoning the race. I was furious with myself as I was doing pretty well up to that point. In the second year I used a gps trace I'd plotted off Google Earth as a backup. Visibility was better that year and I didn't really need it but it was very reassuring to have the 'breadcrumb trail' on the watch.
I don't think GPS watches will encourage runners not to bother learning nav skills; most fellrunners don't seem to be able to navigate that well and I suspect they never have been able to, even before GPS came along. It's simply not what they run for; they run the fells because they enjoy the freedom and the space.
It might be different for walkers; gps might encourage them to go further than they really should but again we're just speculating.