I don't often confirm my suspicions with my partner, but since he's the one that's studied in Japan, I defer to him on such matters. His opinion was exactly what I said... that although they didn't practice them all the time, the Aikikai schools did practice the taisos, just not to the degree that the Shin Shin Toitsu schools do.
You have to remember that Tohei was the chief instructor at the Aikikai, so it's only natural that some of his "stuff" stuck with the organization. Like me doing "old style Aikido" with Toyoda, the hundreds and thousands of teachers out there will teach what they know. If they learn the taisos, they'll use them. They're easy to do, and they work. They're essentially used to teach movement, just like the wrist exercises are used to teach "grips".
... not real complicated.