[A Doug Adams fan, I see.]
I have taken both US Sailing and ASA certification courses and they appear to be roughly equivalent, although each institution, school, and instructor will have specific agenda. To answer your specific questions, do a search for the US Sailing and ASA Web sites and read the info they have on the various certification levels.
In locals waters, someone with a 101 competency "might" be allowed to anchor/moor/dock overnight in a small boat (e.g. 24 footer). As stated, night sailing is almost always prohibited. For larger boats (32'' and up, for example), the charter company will likely want much more experience/training.
If, like me, you were/are not able to get big boat experience with other competent sailors, then I highly recommend good instruction. In order to skipper a larger charter boat for a day or a week (even in a benign sailing ground like the BVIs), most charter companies want to see either a fair amount of experience in boats of the size you want to charter, or at least some formal training up to the 104 level. Of course, I have some evidence and lots of anecdotes that a valid credit card is the major crtierion they use (especially for the smaller, hungrier companies).
When you think that you are taking charge of a $100-300K boat (depending on size and condition) and the safety of your crew, you really don''t want to just "squeak by" in terms of competence, IMHO. If all you have is a week of 103/104 training aboard a 35 foot (plus of minus) boat, you still have just scratched the surface of your lifelong learning process to be a good sailor (again IMHO). The mechanics of handling the boat are one thing, but developing good judgement is another. Sometimes you have to have a few uncomfortable situations to realize that a particular method or attitude is not advisable. We learn from mistakes, and we have to hope that they are not costly ones.
I''ll tell you that my weeklong 103/104 training prepared me very well for the 45'' sloop I chartered for my first bareboat charter last year. I''ll be learning for the rest of my life, but it gave me the foundation to build on and we had a fantastic time.