User blog comment:Rappy 4187/Technical Update: August 31, 2015/@comment-5558012-20150901003221/@comment-1757994-20150901003920

"How exactly is this an improvement? I guess I'm confused as to the point of this. "

The only thing I can think of is if someone with edituserjs stuck something in your personal js without you knowing. Then you can go to your prefs, turn personal js off, undo, and re-enable. For people who have no idea what personal js is, the option would be off by default. That's a pretty specific risk, but I guess it's a little better than before.