At the moment it seems like they are not needed in most places. When i first got the game i was building farms everywhere in an attempt to tech my cities up quickly. I found though that shortage of cash and time are much more constraining on growth then city population. Last game i played i only built level one farms and still couldnt' afford to upgrade all my cities on time.

Did run across a few cities with seemingly low pop growth rates that did require more than one farm though.