"If you have n options to modify the state of your app (where, if you insist on having a toggle for every change to the UI, n is going to be high) then you have 2^n states to worry about maintaining, 2^n states to worry about breaking every time you make any change to anything, 2^n states to make a confused user choose between their settings page..."
"are you seriously trying to tell me that youtube doesn't have the bandwidth to serve two different users different CSS files?"
"...No! I'm not doing that! What the fuck?!"