I use them generously, and I also was surprized about this.
Come to think of it a little, it is probably because the class “two” could also apply to other adjacent classes, so you can define .column.two, but say you also want to define .level.one for the same element… then you will have a problem because it won’t be clear to what the two applies. You will need to do something like .column-two and .level-one.
But heck, i’m gonna continue using them, I guess. Hey after all, I know what I’m doing.