Let's forget all the arguements about whether or not you will ever be attacked while you are wearing your sword and sitting in Seiza. This particular arguement has been done to death.
Let's look at the following question instead. What is to be gained from training in Seiza?
I'll just start this little brainstorming session. Remember my experience is heavily colored by MJER. Your milage may vary.
Strong quads.
Strong hamstrings.
Durable knees.
Good posture.
Reenforces proper tenouchi.
Extra practice on some of the core principles from some of the more advanced sets.
General sense of readiness from any position, even the unlikely.
Lots of built in Kihon practice in general.
Seiza prepares you muscularly for Tate Hiza, which is a more useful scenario.
Feel free to add your own thoughts, this is just the result of a couple of minutes of brainstorming on my part. Keep in mind that the kata are not the style. They are only a training tool to teach you the style. The seiza kata are in the MJER because they are a good training tool to teach you MJER, same as all the other kata. The kata are not a plays in a playbook. They are merely repetitive training exercises.
Charles Mahan
Iaido - Breaking down bad habits,
and building new ones.