Actually, I believe that Seitei Gata was created because our society today pushes us to want to be "tested."
Back when learning to use a sword was a good idea to stay alive (assuming you were samurai, of course), sword-handling was a martial skill, rather than an art. You were tested for proficiency only by your sensei. And assuming you lived through your fights & battles, you pretty well knew that you were competent.
But as we don't wear or use swords today outside the dojo, iaidoka in the sport of iaido had no way to compare their skill levels. So in the 1960s, the "original intentions in creating Seitei Iai was to produce a standard that could used for gradings nationally and later internationally." That quote is from Hajime Baba, by the way. Why do people climb mountains, jump out of perfectly good airplanes, & ski-board down vertical slopes? Because they feel they have to "prove" themselves by living through the experience.
How can an iaidoka test him/herself against others who have learned different ryu? In short, they can't. My wife & I study MJER, while our good friend studies Tenshin Shoden Katori Shinto Ryu - what do these ryu have in common? Other than the basic motions of nukitsuke, kirioroshi, & noto, not much. But when we are tested in Seitei Iai, there is a leveling effect, as we are all performing the same waza.
I agree with Jason that learning Seitei Iai does serve as a sort of apprenticeship in that new deshi are just learning to handle their iaito, but also agree with Kim that it sure doesn't make a lot of sense when you should be building on acquired skills. Maeda-Sensei made the decision to stop teaching Seitei Iai a few years ago, & now teaches only MJER waza. But he also adds in several Seitei Iai & other ryu waza that fit well. For example, he has us learn & practice three ukenagashi - MJER, Seitei Iai, & MSR - because maai progresses from quite far to extremely close, & that trains us to perceive what teki is doing & to react accordingly. And Soetetsuki fits well in Chuden. And so on.
This doesn't really address Alexanian-Sensei's original question about crossover between & among koryu, as much as it does the use of appropriate waza in teaching. I found Seitei Iai to be a lot more confusing than MJER Shoden, Batto-ho, Chuden, etc., but that of course might well be because I was first learning Seitei Iai as a brand-new deshi. I do find MJER waza to be lot more consistent, as per Kim's comments, but am not sure that I would vote to eliminate Seitei Iai, if given a choice.
Ken Goldstein
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Judo Kodansha/MJER Iaido Kodansha/Jodo Oku-iri
Fencing Master/NRA Instructor
"A positive attitude may not solve all your problems, but it'll annoy enough people to be worth the effort."