You make a very good point. However, I think there is something to be said for being able to hand a complete rules set, *for the game you're going to be playing,* to the players. And that's not always an easy thing to do under any version of any system.
'Admit it,' --
Handing the yellow brick of BRP to a player, then saying, "We'll be using options blah-blah-blah-blah, but not options blah-blah-blah, just read up on it," is a bit of a big ask, as well as confusing. Also opens the door to players coming back with, "But gee whiz, options blah-blah-blah-blah look SO COOL, can't we use those?"
I don't have all the answers that are best for everyone, and I think fragmenting the consumer base is usually a bad thing. And I like having the big BRP book. But I can see the other perspectives, too.