Yes, I'm well aware of which rivers have campsite reservation systems and which don't. When the Middle Fork switched to their reservation system, I had my reservations (pun intended, sorry), but I have found that it works very, very well. It is SO NICE to take a lazy morning in camp or really take your time and enjoy the stops along the way downriver when you know where you will be camping that night. It works great for me, anyway.
As for the Main's system, I am not so keen about it since I am almost always in a small group and larger groups invariably bump me out of any reservable camp I might request from the rangers. OTOH, I have found that it's much more fun to just play it by ear and go without any reserved camps. There are many nice non-reservable camps to choose from, including some big camps and camps with good shade.
I've never run the Selway, but with only one launch per day during the control season, I can't imagine there is much need for camp-running. Plus, the rapids are harder, which tends to make groups stick together for safety. Which takes us back to Hell's Canyon, where camp-running was, based on my limited experience, pretty damn common. I'd love to hear if folks are still seeing commercials do it a lot.