It also depends on the opportunity cost of not harvesting. Let's say you harvest a stone and get a trader X turns earlier (depending on where you would produce the trader if you didn't harvest). Now on top of the harvested production you get X times the income of the trade route. Plus some of the future stuff on your agenda will kick in sooner (e.g. the city which was supposed to produce the trader, gets its campus X turns earlier; then the next thing earlier etc.). The negative opportunity cost is mainly the builder charge used for harvesting. I mean 100+ cogs right now VS +1 per turn is usually an easy decision. Maybe harvesting is too strong. And while it makes sense to "cripple" a peripheral city for the good of the empire, it isn't necessarily fun. Basically you are also encouraged to "pillage" your own cities that would that would take too much of investment and then only contribute marginally. Scorched earth so to speak. I feel bad for the poor little city that only has some plains and tundra left, because everything has been harvested. One thing which annoys me in particular, is that I have to remember to send a builder to the location of a planned district many turns in advance, if I don't want to waste the underlying resource. And then I have to speed up something else with the harvest, because the harvested production is not stored and does not overflow. Quite the headache sometimes. While the rationale for the harvesting mechanic is perfectly understandable, all the implications and trade-offs aren't necessarily fun!