So why would you want to have cities with 6+ grain then..? (e.g. I now have a city that has 10 grain due to farms and such)
You can reach level tresholds much faster and cheaper. (no need for "farm" buildings) It's more of an issue early in the game, later high production and tech will solve it for most 2+ grain cities. For instance, a 6 grain city can reach lvl 3 quite fast (Oracle/Scroll Scribe, Infirmary/Strike Garrison/Watchtower improvements), and the upgrades you get there is quite nice 