IMHO, I think you need to be relatively proficient at fly casting, to be proficient at catching fish. I'm not saying you have to cast 100', but I think if you can only cast 30', you'll be limited to what situations you can catch fish. I used the 30' mark as an example, since it is often used as the distance most trout are caught at(or under). The waters I fish for trout, you'll be at a disadvantage if you can only reach 30'. 50' will open up more opportunities. More than that, you can reach those "impossible" fish.
Line control, and presentation is a big factor in catching trout, and other species, but I have not found where a good caster is seriously lacking in those departments. It's more the inexperienced casters. The guys I know that have the best line control are good, to very good casters. They can also present a fly in all sorts of situations, and conditions.
I used to practice weekly, but now I just don't have the time. I do try and critique my casting while fishing, and make improvements. I recently found some little tweaks to help my casts, then I started having some inconsistencies. Last trip I worked through it, and I'm back in the groove......or was. Who knows how that plays out this weekend.
I have not spey cast in a while. I hope to get out an practice/fish soon, since I have an upcoming trip. On that trip, I will ask the guide to help with my spey casting.
A big part of the reason I enjoy fly fishing, is the casting. If it was just about catching fish, I could do that more effectively(for the most part), and with less effort, on conventional casting gear. Fly casting is a challenge. I think that's why I'm drawn to spey now. It's a challenge to learn, and catch fish with this method. I tend to get bored with things, if they get too easy, or repetitive. Fly casting is something that never gets boring for me, and there's always more to learn.
briansII