Green being better than brown for production isn't even just a slavery thing.
e.g. throughout much of the game (particularly when you have the bonuses to make workshops reasonable tiles), you can generally trade one food for up to two hammers by adjusting your tile improvements, so starting with a 2
base tile gives superior results to starting with 1
1
or 2
.
Or even early game; a food resource and 5-6 grassland hills makes for an early game production powerhouse (especially if there's a strategic resource thrown in). Plains hills are much less effective.
Plains are great to have when you have too much food. But gameplay has developed to the point where it's hard to actually have too much.