I think the reason why these sets have stuff we've seen over and over (such as Rubber Duckie, C is for Cookie, etc.) is because these are catered towards casual fans as well. In other words, people who have been longing to see this stuff. This is something I've noticed on Muppet Central, and even other message boards devoted to other classic TV shows. While most people out there like the stuff that was more common, it would appear that we always prefer the stuff that was more rare. And unfortunately, that usually ends up being what may never be seen again.