Asteroids is good for production too.

Later upgrades to the farms make them +2 or higher per tile. Additionally, you can use the Food Distribution improvement for a % bonus on an agriworld. Alternatively, those isolated tiles on low class planets are good to use for farm tiles as the lack of adjacency bonuses make them less viable for research centers or markets.

^^Yeah, that's what I do. Currently, improvement buildings are far too weak. I build almost no factories/labs/markets. I pick about 10% of my planets to be food planets. If I have a good food planet, like a class 12 breadbasket world, that planet can feed almost my entire empire. I spam nothing but farms, Kimberly's refuge, and I'll settle a bunch of farmer citizens on it. 

As for my other cities, I'll give them one or some of the starter buildings, like the core mine or space elevator, for example, but beyond that it's mostly cities and morale and colonial hospitals. I do build the major buildings, like financial capital/mfg cap/research cap. 

If you have to even question the power of cities you have no idea what you're missing. Focus on growing your population and your economy will dominate everyone else. 

Is anyone abit annoyed that cities cannot upgrade or be destroyed? I didn't think much of it until I annexed a planet to find it was nothing but cities. My food and approval dropped like a rock and I think my food went negative (did keep playing long enough to check), my only option was to destroy the planet figured I could recolonize it and use it but it turns out when you abandon a planet it somehow gets reduced to a 0 dead world.