It's required in CT, NY, NJ and other states. Here in CT we can't register a boat or operate one over 9.9HP without a boater card. In CT it's a lifetime card, so you only need to take the test once and there's no need for renewal.
Based on some of the behavior I've seen (especially among power boaters) I'd like to see more training required. As of now it's a one-day course and they test you at the end of the day. If you don't pass they tell you what you got wrong and then re-test on the spot. At the very least I'd like to see them make it harder for the flunkies who don't retain the info (have them take the whole day over). That would make them pay attention better. But I think a mandatory 5-year online refresher training about rules of the road, navigation aids and lighting, and emergency situation handling would also be a good thing. I took the US power squadrons course years ago when it wasn't mandatory. That was much better, and more thorough. I encouraged my wife to take it when we got our first boat as a couple, and she did. She thought that was worthwhile.