If it was actually about helping the user, there wouldn't be an opt-out link, because the first instruction in the new "self-XSS" instructions is going to be "go to this page and opt-out - yeah, it says a bunch of scary bullshit but that's just because they don't want you to know about $feature!"

The opt-out is there to boil the frog until they can remove it in the guise of "security". I also agree with bsamuels that it provides a convenient CFAA lever to hit in the event you do run a script they don't like. ("They had to deliberately bypass our 'protection' to paste the javascript into the console!")