Yeah, I almost included that. I kind of forgot and kind of considered it a can of worms, because at first sight the slider's "shotgun" nature can make it seem inefficient. So in short: it's not inefficient. Use a low level for the entertainment slider (or the "luxury slider", as us Civ I/II old-timers keep calling it), and you'll find most cities that don't need entertainment won't spend on it. If you ever become a micromanagement freak like me, you'll avoid some of the excess spending by removing military police from "over-happy" cities, making them grow or shrink so their spending is appropriate, etc. But keep in mind that, while while gold is very important in the long run, it's often just left sitting there when you're in Despotism (and can't gold-rush), so often the best way to "spend" it is to "trade" it for food, shields, and flexibility by using the lux -- er, entertainment slider.
(Its mechanics are: X percent -- the amount you set -- of each INDIVIDUAL CITY's gold generation will go into creating happiness, with each gold spent generating 1 happiness.)