Thanks for the replies - I, too, wish that they had the Amateur Obstacle classes, because those are my favorite classes, and I might show at R Nationals next year. It does make sense, though, that they are left out due to time constraints.
Maybe AMHR should consider showing in two rings at once to save time. That's what is done at AMHA Worlds, although only for one day, and it seems to work quite well. After the youth and amateur classes are finished in the main arena (the end of the day on Tuesday) that group of judges stays in the main arena to judge the futurity halter classes on Wednesday. The open group of judges start on Wednesday in the second arena to judge the open showmanship classes and the individual work performance classes (Hunter, Jumper, Halter and Driving Obstacle). Then, on Thursday, the open judges move to the main arena to judge the rest of the open classes (driving and halter), and the youth/amateur judges are finished. This method works quite well, and saves a lot of time. It helps that the show management is really accommodating if you have a conflict.
Also, at AMHA Worlds, the Youth Performance classes are split into 12 & Under and 13-17, and it seems to be pretty fair that way, because you don't have the small kids competing against 17 year olds.