I have observed that more ordinary snakes that have not developed a rattle often vibrate their tail in a similar manner, which often makes a warning buzz that is merely somewhat quieter than a rattlesnake's rattle. So incremental improvements to this rattling mechanism, which started with a regular tail, would just slowly increase the loudness, and thus warning ability, of a snake's tail.
Of course I agree with you. I am merely thinking in dollars and cents here, since that is the primary measure of value in the "civilized" world.