Personal story:
1981, on or immediately after my 18th birthday, I register with Selective Service, at the post office. Small town, my uncle was the Postmaster, and I had not yet questioned such things.
Within a month I graduated high school. I entered university, was commissioned as an officer in the U.S. Army through ROTC, and graduated. I entered active duty in 1986, graduated Officer Basic Course, and held a TS-BI security clearance. I was stationed overseas on a Nuclear Release Authentication System team, subject to the Personnel Reliability Program. I was also paying off my student loans
I was home on leave visiting my parents in 1988, and a letter arrived from Selective Service. They informed me that I had not registered as required by law, and therefore I could not get any student loans, get a federal job, or hold any position that required a security clearance.
lolwhut???
Then I went on to get a federal job after leaving active duty.
Another story: A friend of mine actually served time for not registering. They had no problem finding him to prosecute him. If the purpose of registration is to find registrants in order to draft them, then isn't it rather pointless if they can already be found?