For what it’s worth, I have a slightly different perspective. I only fish a couple of patterns 95% of the time. I like to fish a very generic looking fly, something that looks a little bit like a lot of different things. As long as it more or less looks like something good to eat, fish will give it a try.
I will fish a specific looking fly if there is a blanket hatch going on, but at least where I fish, that tends to be the exception, rather than the rule. More often than not, there are a number of different food forms in and on the water, and because of that - the fish tend not to be overly selective. Of course, you mileage may vary depending on where you fish.
One of the couple of flies I fish is a pattern shown to me by Carl Richards, co-author of the book Selective Trout. It’s a pattern called a Chicago Leech. Mr. Richards said it was the most effective wet pattern he had ever fished, and he fished it often, over many of the more exacting patterns he had developed. I looks a little bit like a lot of things – minnow, leech, stonefly nymph, crawdad, etc.
The one thing I will add, I find it much more important to try a wide variety of presentations, I very rarely fish a fly dead drift. I change presentations often, sometimes on the same cast, I very rarely change what fly I am fishing. Again, this philosophy may not work for everyone, everywhere; but it does work well for me.
Have fun,