Back when I first started fly fishing for trout many years ago, 6- and 7-weight lines were fairly typical. Many anglers liked something a little lighter and went to 5 weights. If you really wanted something radical, you went to a 4 weight for a refined approach on difficult trout.
Rods were soon destined to get ever lighter. First came 3 weights, followed by 2-weight rods, and finally, the ultimate lightest rod possible was unveiled when Orvis released a 1 weight back in the 1980s. Many traditionalists were skeptical. Wasn't this going too far?
Actually, no it wasn't. If you held back your skepticism and tried one of these 2 or 3 weight rods, or even the 1 weight, they proved to be efficient fishing tools. The catch is they had to be used for what they were designed for — short range fishing with light leaders and small flies.
Besides trout, they also proved efficient for catching small panfish, such as bluegills, rockbass and shellcrackers.
They increase the challenge of the sport and are less fatiguing to use over the course of a day. They almost feel like a piece of fluff in your hand compared to a regular 5 or 6 weight outfit.
My conclusion is that the 1 and 2 weights are a blast to fish with and fun for an occasional outing, but not a "standard" rod that I'd turn to for everyday fishing. The 3 and 4 weights could well become your regular go-to rod if most of your fishing is with size 14 or smaller flies on tight quarter streams with lots of cover along the shore.
Their lightness makes them a joy to fish with. Arm fatigue is a thing of the past. The line lands on the water delicately, so their especially good for super clear water and skittish fish. The thin fly lines are more supple and flex better with currents, causing less drag on the leader and fly.
The short casting range they are good at — out to 40 feet or so — forces you to wade more cautiously and sneak in carefully for a low-profile stalking approach and more accurate delivery. Finally, I simply like the beauty and almost magical feeling of casting these refined outfits. Fly fishing becomes a game of rhythm and timing, not brute strength.
If you want to cast far off across a pool to reach a fish that you can't wade to, these light rods aren't up to the task. You'll have to circle around and try for the fish from the other side.
Casting heavy weighted nymphs or large streamers is difficult with these outfits. Wind also makes it harder to cast with these light lines. A heavier line will cut through the breeze better. Leave them home on windy days or try to cast with the wind at your back. They're also shorter and offer less leverage for landing big fish.