If you're thinking about going to college and applying for federal financial aid, they won't give you any money if you do not register with the S³ (if you qualify to get any money from them anyway). That's the only thing I know offhand, but I know plenty of people who have not registered with the S³ and nothing bad has happened. I'd reckon there isn't much need for the S³ now though, since there are a lot of people who are willing to be soldiers. This would also be true if the United States would mind its own damn business and stop meddling into other countries' affairs. I guess the S³ is like a fallback plan in case the United States pisses off every country on Earth and they decide to come gangbang us and they need all the males in the countries to come fight their war for them.
Jam it back in, in the dark.