I have much the same experience that Larry described. In his OP, he indicated that he was talking about trout rivers, not steelhead (I know they are trout, too, but...), and that his perception was that rivers that often fish better are not heavily guided. I fish in Idaho and/or Montana every summer, and have sampled, sometimes pretty extensively, most of the well-known and many of the not-so-well-known rivers there.
On its face, this presents a conundrum. A guide's business is to get his/her client into fish. Ipso, facto, they should be taking them where the fishing is best, right? But that isn't always true. Hence Larry's (and my) impressions that the rivers that fish best are NOT frequented by guides.
I think there are a couple of explanations that intersect. One is that there are some rivers where pressure from guides is a significant factor in making fish more wary and, therefore, harder to catch. Of course, pressure doesn't come only from guides, but on most rivers, guides can more consistently find fish than can the tourist fisherman on a DIY trip. So, us poor schmucks who visit on our own suffer the consequences of trying to catch those wary fish (oh, wait a minute, isn't that the epitome of fly fishing? Yeah, but we still like to catch a fish now and then...).
So, why then are there still rivers where the fishing is great (which means the catching is more frequent); why aren't the guides taking their clients there? I think this gets at the point several of you have raised. Those rivers typically are the ones that are not conducive to fishing from boats. Not all, but almost all guiding for trout is done from a boat. Clients want it and expect it. Similarly, the increase in recent years of privately owned drift boats means those same rivers are infested with both private and guided boats, sometimes so thick a boat is never out of sight of the wading fisherman.
So, I think the bottom line is that easily floated rivers, which includes some of the most famous rivers in the west (thus the ones clients from out of the area expect guides to take them on) are the ones that get hammered the most and make the fishing a little tougher for everyone.
I suspect that Larry is like me, a wading fisherman, who prefers dry flies to rising trout. Sure, it might appeal to snobs like us to have rivers where nymphing is off limits, or guides are banned (intermittently or completely or via lottery), but I think the real factor comes from the quantitatively greater and spatially more distributed pressure that comes from rivers with lots of floating fishermen. Heck, some of the best rivers I know are not restricted to fly fishing (so nymphing restrictions won't do it), or even to catch & release.
My life's too short to worry about trying to change the rules to favor the style of fishing I prefer. I'm just grateful that so many pay-to-fish folks are too lazy to wade and fish and that so many people today want to emulate guides by having their own ride. Let 'em have the rivers that fit their style. I'll just continue to find the rivers they shun and continue to enjoy the experience.
D