Is there some analysis behind that consensus though? My experience so far is that if you're going for the legacy paths each age, you need to settle / capture enough towns to get the military path points and the resources for the economic path. As the age progresses you get more gold to convert towns to cities, but the cost increases each time and at some point, when the age is close to an end, you won't have time to develop the new city sufficiently to justify the cost.
So briefly, I doubt it's a good idea to convert most of your towns to cities because at some point, for the cost to upgrade the town and build the infrastructure, you could get more science/culture/production yields faster with a food town and converting that extra food into specialists in one of your cities.
We'll see if the faster growth curve will lead to feeding towns becoming less important to grow cities.