A pre RP windmill is +1

, congrats you've just converted your hill into a brown farm. Rather than growing if that is my best tile to work, I will almost invariably work a specialist. This means I get a better

->

conversion from whipping. Early in the game I try not to settle in locations where I can't have substantial for surpluses (those can be backfilled later). Even worse, if I'm at the healthy/happy cap I actually get less from more food; the value of GPP has not dropped down yet.
Several points:
GPP are useless in cities that will never pop a great person. Less than half of my cities usually pop a GP because some go at such a fast rate. So you are then running a specialist for the flat bonuses it gets. +2 hammers from an engineer, assuming you don't have representation.
You run your engineer in your smaller cities, I'll run my plains hill windmill (1 F, 3 H, 1 C, even pre replaceable parts, 1 F 2 H 1 C is better in some cities instead of 0 F 4 H) along with half of another tile provided for, which you can add to the effective output of the windmill.
I think people underestimate the effect of one commerce. It is small, but it adds up. You can't ignore it when determining the value of windmills.
And at a certain point you will most likely leave slavery. This increases the value of having many tiles worked.
Late game I'd normally rather run fewer windmills and more watermills/biology farms for production (windmilling over the mines in commerce cities only after I have all my desired infrastructure, unless I decide I need lots of crappy military production).
Farms are inefficient when it comes to maximizing a cities raw output, meaning base commerce + hammers. This is because with more farms you reach the health limit quicker, at which point the only output you get from working each 4 food farm is one food (2 food to sustain the farm, 1 food for sickness, equals 1 extra).
The only cases where I would usually spam farms like that is when I am trying to build more specialists(either in representation, or GP farm), or if I am focusing on production(through hills or increased slavery rate)... or increasing votes.
Also, consider the case when you don't have access to fresh water or rivers, and don't want to chain-irrigate over pre-existing improvements. In that case you can't utilize your farm/watermill strategy. Regardless, it is a somewhat manufactured conflict, as watermills and farms don't utilize the same tiles. They can't be built on hills.
You may think any city that can't build farms/watermills is useless, but a size 10 city with lots of hills, sustaining steady growth through windmills, is a useful addition to an empire imo. The windmills can be used in a coastal city with little production or surplus food... even a 2 F 1 H 1 C tile (pre Replaceable parts grassland windmill) is a useful addition to an otherwise coastal/barren city.
Windmills are the absolute only source of extra food for hills. I think the value of them can largely be summed up with that fact.