Conventional wisdom states that "it is a 2-for-1" so you may as well fish the dropper to increase your odds of catching fish. I am not sure I generally agree with this conventional wisdom. First, I do not think that it doubles your odds as you generally do not fish either fly quite as effectively as you would fish either one by themselves. What I mean by this is it is hard to cast that dry fly right next to that log or undercut bank with a dropper is hanging off the back a foot or two. Nor do you get the nymph down as deep as you would always like nor are you in contact with it as well as you would be with a tight line nymph technique. And maybe most importantly, in the best of cases, it is a mild pain in the ass to fish with a dry and dropper and often a much larger pain.
Where Droppers are Advantageous
I am not anti-dry and dropper and I have done very well with them at times. I find that by late-April into May, "dry and dropper season" begins and it really can continue through the end of the season, if you are so inclined. I am usually not so inclined but you may be or maybe you just want to figure out if it is for you. Early on, the standard Driftless rig Andrew Grillos' Hippie Stomper with a small beadhead dropper hanging off the back of it. I have had great days on this rig. As grasshoppers and other larger terrestrials get going, larger dries like "Chubby Chernobyls", Morrish Hopper, or other larger foam creations are the flies du jour.
Figuring things out - A dry and dropper is a great way to figure out what fish are keyed into that day. I will fish a dry and dropper when I am not sure that they are going to be hitting my dry flies which are what I would prefer to be fishing given the choice. I am not a big fly changer - I just do not think that fly patterns matter all that much - but I will change my dropper from a caddis to a mayfly or cranefly imitation to see if that makes a difference.
When they are not really into any particular pattern - I will stick with the dry and dropper when the fish are hitting around 60% / 40% or so on the two offerings. There are certainly days where a dry and dropper makes a ton of sense.
Fishing Riffles - I hesitate to give this one away. Many anglers pass up some of the most productive fishing - the riffles - so they can jump from pool to pool. Pools are great but when there are fish in the riffles, they are almost always actively feeding (see the post on optimal foraging theory). Fish in riffles tend to react quickly before food passes them by so a visible dry and a dropper works great. And generally in riffles, there are so many little hiding places for trout that making systematic casts to cover the riffle works great to find actively feeding fish.
Pool hopping - generally this one pretty much goes with the next one. There are times when jumping from pool to pool - not that you can't fish the areas in-between - is how to best catch fish. These pools are often pretty "clean" which means you do not have to worry that much about fishing tight to cover as the fish are often related to depth rather than instream cover.
Fishing the thalweg - the thalweg is the basically the area of greatest flow in a stream. Fish generally do not hang out right in the thalweg but will be just out of that main current where they can wait for food without using a lot of energy to hold position. Because the thalweg has a lot of flow, it is pretty easy to have your flies hit the thalweg and allow it to carry them until it is time to cast again.
Basically, where I like the dry and dropper combination of where fish are not holding tight to cover like undercut banks, woody debris, or mid-current obstructions. I have had really good luck with dry and dropper where riffles dump into pools and the seams along the thalweg.
Where I Do Not Use Droppers
I think that the conventional wisdom that it doubles your odds but I do not fully buy the logic. I have no idea what the real increase in odds are in the best of situations. I am fairly confident that there are times that the dry and dropper decreases your odds compared to fishing one or the other in a more effective manner. If I really want to fish a nymph, I am probably doing it on a New Zealand indicator or other bobber or I am fishing a tight line. And if I am fishing a dry fly, there is a pretty good chance that I am twitching and moving my dry fly at times. Both of these are tough to do effectively on a dry and dropper rig.
When the fish are really on one pattern - there are days when fish are smashing my dry fly or my nymph, but it is strongly one or the other. I would rather fish that
When they are hitting the dry fly "well enough" - I would rather fish a dry fly and some days, well enough is good enough for me. (see the last point in this list)
Fish are tight to cover - there are times that fish are tight to cover and having a dropper a foot to three behind the dry fly means it is hard to control the cast well enough to fish close to cover. There are times where being three inches closer to the bank matters.
Fish are deep - there is a bit of limit on how long your dropper can be and be cast and fished effectively. If the fish are hugging the bottom and not willing to move, you might have to get down to get to them.
When I want my flies to move - I think too many anglers get too hung up on this idea that your flies need to dead drift. Watch the bugs and tell me how often they are sitting perfectly still. In particular, our best hatches, the caddis, craneflies, and terrestrials (if you want to stretch the definition of a hatch) move - a lot. And our streams are filled with scuds which are active swimmers. I find that I can move my flies better when it is just a single fly.
When in-stream weeds are just too much - there are time that it is really hard to fish subsurface on some of our spring creeks and fishing a nymph is just too much of a pain in the ass. Elodea will grow to the surface and the flow will be restricted to tiny fraction of the channel but fishes will be sitting in the weeds watching for food moving through this channel. Casting accuracy is at a premium and hitting your spots with one fly is so much easier.
When it is too much of a pain in the ass - Let's face it, it can be a huge pain to fish two flies. If I make a hell of a tangle - it happens to us all - I will quit fishing two flies and go with what is working best. When it is really windy or I'm trying to make casts close to a bank with overhanging vegetation and things are not going so well, it is time to make a decision which fly to fish. There are days where I am trying to fish a 3 or 4 foot dropper and even with opening up my casting loop, it is really hard to control the cast well enough.
I am most certainly not anti-dry and dropper but, like most everything, there is a time and a place where it is more effective and other times where it is counter productive. There are times I do much better with a single fly fished efficiently and other times where a dry and dropper provides better fishing. It is all part of the experimentation and learning experiences that is fly fishing.
Just something to think about - what are your thoughts on the Dry and Dropper rig?