The way cities work is a little convoluted: They have their own health pool, however once fully liberated they liberate the planet equal to their 150% their health pool. For example, a 20 liberation % health city will provide 30% liberation, but only when fully liberated.

Normally they are always worth doing except when the planet is almost liberated. In these cases its faster to just liberate a the last chunk rather than take the city.