In Civ5 there are now three ways to proceed with captured cities: raze, annex and puppet. Why not add the possibility to occupy the city?
Occupying would not really add the city to the empire - there is no impact on research cost, social policy cost and so on. You get the gold from capturing and have the possibility to plunder great works and you have acces to ressources on the captured land but you don't get anything else from the city (e.g. gold, science or bonuses from captured wonders). In return, occupying does not cause warmonger penalties or liberation bonuses nor does it instantly create unhappiness like annexing, puppeting or razing - so you gain time to continue your war but you have to leave a unit as garrison in the city and the unhappiness is gradually increasing the longer the occupation endures (contrary to annexing, puppeting or razing where the unhappiness is decreasing over time). So you have to deal with your occupied cities sooner or later - it's not a long term solution.
The possibility to occupy a city should give you more options in war and following peace deals.
Occupying would not really add the city to the empire - there is no impact on research cost, social policy cost and so on. You get the gold from capturing and have the possibility to plunder great works and you have acces to ressources on the captured land but you don't get anything else from the city (e.g. gold, science or bonuses from captured wonders). In return, occupying does not cause warmonger penalties or liberation bonuses nor does it instantly create unhappiness like annexing, puppeting or razing - so you gain time to continue your war but you have to leave a unit as garrison in the city and the unhappiness is gradually increasing the longer the occupation endures (contrary to annexing, puppeting or razing where the unhappiness is decreasing over time). So you have to deal with your occupied cities sooner or later - it's not a long term solution.
The possibility to occupy a city should give you more options in war and following peace deals.