They tested two cities and found the one they gave fluoride to crashed in number of cavities. Politicians fell all over themselves to introduce it everywhere, so they could look like the heroes.
I would be stunned if it had a secondary side effect that was beneficial to Worldly Overlords.
I listened to a "How Stuff Works" podcast on fluoride (stuffyoushouldknow.com) and they made fluoridation sound like a bad idea and even hinted that it was mildly conspiratorial that it's in our water. I was surprised to hear it as the guys normally come across as pretty skeptical.