I, also, wouldn't keep a horse I couldn't take out on trail rides, especially if I couldn't take them out alone. I live in northern Colorado, about 1/2 mile from two different lakes, and right now a lot of what I do is go out and take my gelding for long rides. Not only that but when I compete I event, which means going cross country for a couple miles alone. Granted, it's not like it's overgrown and I never see people (as there is a judge at every jump), but there are definitely areas where a horse that didn't enjoy (or at least tolerate) being out of an arena alone would not work out for me. Personally I've never met a horse that didn't enjoy trail riding once they realized they weren't going to be attacked by a cougar (or some other silly notion they seemed to have in their heads). I've worked with plenty of horses that were considered "too flighty" for a trail, and all it took was some patience and calm riding and the horses were fine. More forward than your average trail horse, I would suppose, and a little more prone to spooking, but they settled in well, and if the rider is paying enough attention they can generally tell when the horse is going to spook. Of course there are exceptions, but I think there are very few and that most all horses would benefit going on a trail ride once a week or so. Or at least getting out of the arena.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."