Whenever I am out on the water and decide to cast really far, I find the trees behind me are much closer than I thought they were. Then when I finally do get a nice long cast out there the current puts a giant belly in my line and yanks my fly up off the bottom. I don't catch many fish this way. It is nice to have a lot of line out to strip a streamer upstream or in still water. I'm still usually relatively close to where the fish actually are. They can be feeding really close if I am stealthy. I'm not so stealthy trying to cast 90 feet.
I've never measured how far I can cast. I have cast all my fly line out on my 9wt. That was in great conditions in my yard. I usually cast plenty far enough to catch fish though.
I have one tip for ya'...never ever expect a single definitive answer to a question on this forum!