RJ,
Most RB divers are diving a bit more than just recreational, as such their specific mission (dive) needs vary depending on the dive. For instance, a team of divers going deep in to caves could have a very different set of requirements than say a buddy pair going to do a photo shoot with dolphins. Add to that that rebreathers are still relatively new in the mainstream and the various agencies have not yet developed guidelines that have enough data to support a particular philosophy (general guidelines). Each of the agencies right now that support RB training have an independant root of base knowledge developed from ad-hoc experience in their particular diving interest. Now I don't mean to say this is not thought out, only to indicate that there just is really not a "proven" this is how you should setup and dive an RB. Add to that the individual that is currently your 'typical' RB diver. In general they are your above-average skill level, generally well educated, a certain level of affluence (RBs cost a lot) and most of these folks are risk takers. Maybe not crazy type, but measured risks. All RB divers right now are pushing the state of the Art and are pioneering the technology. Depending on what numbers you want to believe, there are only about 10,000 RBs being dived on any kind of regular basis across this planet. These forums and the training agencies are pretty much the only link between these divers. So not a lot of cross training and subsequent cross education is happening yet. Most of the manufacturers are very small garage type operations that can not afford to do the type of testing required to provide guidelines from an implicit set of data. And finally the RBs on the market right now vary significantly in their bouyancy characteristics so one set of "rules" is probably not the best for every type of unit.
So as ScubaMiamiDad said, there are diffierent methods being used and the training an individual takes covers the various issues. A RB user must take a course for each type of RB he/she wants to dive.