In the early days it made sense. To keep weapons tech out of the hands of people for they may miss use it. They still traded with people but never let them join. Seeing as how they were so superior to the other factions (Enlave not included) they saw no need to bring in others.
New Vegas has shown use that this way of thinking has not stopped people from gettting weapons and they are no longer the most superior faction around. The Mojave chapter is slowly dying out and its possible that the main BoS back in California is as well. Their need to get tech as made enemies of pretty much everyone.
Others like the http://www.youtube.com/watch?v=GtXhuaTcohY were the first to want change and they were sent on a mission many knew would end in disaster for their troubles. They ended up creating the MWBoS. The only BoS to let in mutations
Then there is Lyons, like the MWBoS but not as "fascist."
Anyways.. to sum up. Their ideology was good in the dark years after the great war but in the end will cause their down fall unless they change.
(And their is not people out there who would still not misuse it? Not to say your wrong, or that they do not need to change, I agree on your points( well, maybe except the CWBoS one, but that is another discussion we can have later), And i know they have to change most definitely, But there is room for change, while keeping their original mission, because at least IMHO, their is still too many people out there who would sooner try to use a cache of Energy weapons to carve out a empire than use them for peaceful purposes, and i would rather see them in the BoS hands than anyone else at the moment.
Edit: and i would want to join the CWBoS, right now as of 2281, i think they are in the strongest position of all at the moment.)