My personal experience confirms Sg's suspicion that nymphing allows a fly angler to fish a wider range of holding water more effectively than a swing setup alone. Most of our steelhead rivers have only a limited number of runs/tailouts that consistently hold fish and are structured so that they will swing effectively. Depending on the river, there may be literally miles between those most ideal of holding areas, and there is generally a lot of different water in between that holds fish that can be reached, if only by less civilized means. This, in my mind, is the best argument for learning to fish "nymphs," and it is why I usually carry both a swinging and a nymphing setup with me on the river. There is a decided downside to this rule in that it requires me to do a lot of backtracking to pick up the rod I didn't use in a given situation before I can move on to the next spot, but it's a price I'm willing to pay for a better chance at success on a given day.

No matter what your preferred approach may be, your biggest keys to success (with credit to others' comments) will likely be finding fish that are in water you can effectively fish, presenting something (anything?) the way they want it, and maintaining good knots and sharp hooks. A lot like fishing gear, really.