I think it's important to take a certified course, as well as get a lot of experience.
For myself, i know i'm new. I started sailing 8 months ago. I took my Basic Keelboating Standard, read 'The Sailing Bible', 'Sailing For Dummies', and 'The Complete Sailor'. After that, I logged over 150 hours this summer cruising and racing my club's fleet of J24s. After one season, i'm quite capable of safely single-handing a small keelboat. I also know that I'm not familiar enough for the 30'-plus yachts yet. That's what I'm expecting my Intermediate Standard to help with.
The summer's cruising/racing experience taught me much more than the course did, but the course went through the safety aspects that are necessary and often overlooked if you just buy a boat and cast off. Similar to taking Drivers Education before getting your driver's license, you'll be fine 99% of the time but might need that special training for the dangerous 1%. To me, the danger of not knowing what to do in an emergency justifies taking formal training.