I think the BAA is balancing accessibility and exclusivity reasonably well. Based on BM application rates, I would suggest that at least half of the top 10-20% of marathon finishers are aware of what a BQ is and want to run in Boston. And local marathon race directors certainly use the "Boston Qualifier" label pretty liberally on their website - which I think further stimulates interest both in their race and in Boston. Yup, the BAA has bottled a great deal of cachet here - and, although they could probably do what they want, they are smart enough to just tweak the rules every once in a while.
As for the part that the standards play in this, I think it's interesting that they are just tough enough that only about 10% of marathon finishers can boast that they BQ'ed - leaving at least another 10% of finishers knowing that a "reward" is within their sights provided they train a little harder and/or get better conditions on race day. Raise those standards too much and that group that is now 10 minutes off a BQ might just give up. I know many who have - and it's just one more reason they have for not amping up their training. It is those who make subsequent attempts that are good for race organizers everywhere. Not to mention running stores, etc.
I am fairly new to running and to marathons. When I first took up running 6 years ago, I hadn't even thought about running a marathon. Next April, will be my first Boston and now my sights are set on running in other majors as well.