I disagree. It's been a while since I've played a "regular" game with actual rapture, and it's a setting that I miss in our GT games. Some argue that it's abused. An experienced player knows how to handle it properly, and those that don't - well, they have to learn
In all seriousness, there is an other setting that is often over-looked with Granaries. That's the effect of Negative city growth. When a city *decreases* in size, it will also have 1/2 of the size of the total granary available (instead of zero food stored). Of course, that's outside of the Civ2Civ3 ruleset (which only grants 10 food out of either 30 or 40).
The reason I mention this is because of a strategy where ALL resources are dedicated to MAXIMUM science to quickly gain techs, at the detriment of city growth. I've seen this used by Stee and others, and it's quite effective. But you need the granary for this strategy, otherwise you can't sustain it for long. Therefore, selling the granary seems logical at first. But taking this strategy into account, if you decide to pursue it then you'll need a granary in those cities that are decreasing in order to maximize the amount of time you can research techs.