I like the 18-200 simply because I don't want to miss anything. Last year I was hiking in Wyoming and knew I'd want something wide and almost put the kit lens on but was LAZY. But 5 minutes into the hike I come around a corner and there are three big, beautiful moose. I was really glad I had the long lens. Conversely, two years ago above Lake Isabelle I came around a bend, thought I heard a horse whinny and stopped to look and it was a BEAR sleeping 6' from the trail. Well I hustled up the trail and up a ledge and positioned to take a shot with my kit lens (17-55). Not real impressive pictures compared to what I would have gotten with the 18-200. The rest of the day you're shooting really wide.
Honestly, if you're doing nature shots early or late (the best times), you're gonna want one of those fast 50s.
But that is the argument, get good glass and you'll have it with whatever you're shooting in 2015.