I disagree. The agricultural trait doesn't just allow one's cities to grow faster. For players who use workers throughly, it allows one to produce an extra shield or 2 at size 12 in each city. Cities don't grow past size 12 in most of my games at least, or if they do grow past size 12, it's after the wars are over (in a histographic game), it's basically only in one city (20k game), or the extra food doesn't do so much anymore (in a space game), as I need so much food to fill the box past size 12. If I use disconnect-reconnect, the agricultural trait gets my cities to 10 or 15 shields faster, where I can level off growth if I like. So, I view it more as a production trait, as it gives you faster production, and it probably takes a few turns off of improvements/units in each city.