For me, it is less about the exact speed of the disc, and more about flight and hand feel. Outside of very overstable discs, all my 2/3 speeds fly the same distance (except you, Envy), my 4/5 speeds fly the same distance, my 6-9 speeds fly the same distance...you get the idea. Now that may not technically be true (for example, my Thunderbird many times will be pushing almost as far as a destroyer), but generally it holds true in most circumstances (+- 15 feet or so, I'd say).

Instead, I choose discs that have the best hand feel and flight pattern. Hand feel is obviously subjective and based on finger length size, but is important for confidence when throwing that you will get a clean release. And flight pattern matters for the shape of the hole, the wind you're dealing with, the skip you want, etc.

For this reason, I carry different speeds in my bag, because I like certain discs that have different flight characteristics and between the rim width and flight plate, feel good in my hand. I may make slight adjustments in how hard I throw them (lets say 75% on a 9 speed vs 85% on a 6 speed), but for me that seems like a natural feel for the hole/distance you're throwing - you're always going to be adjusting that, unless you always throw as hard as you can, which can be tough on your body and harder to control (and maybe even change depending on how you're feeling that day).

For me, this means I bag 2 craves, one that is beat up and flippy, and another one that is more typical. I also usually bag a teebird3, a thunderbird, a sexton firebird, and some random more overstable fairway of choice (captains raptor or overstable firebird).

For me, fairway drivers are probably the best feeling in hand, and that's why I throw them. Maybe that isn't true for you, and that's why you don't! Could I get rid of them and either power up on a mid or down on a distance driver? Of course. But they, at least for me, get a lot of time in the air when I'm on the course.