Quote Originally Posted by Didz
It reduses city populations back to manageable levels and gives a real boost to income from those cities for years.
It doesn't give a boost to income. It just shifts the unit upkeep costs and wages to other cities you control. If you mean you need a smaller garrison and don't need to keep using low taxes, then I agree with you there.

The reason exterminate is so good is because you have high unrest and culture penalties when you get newly conquered cities. The population boom bonus, decreased squalor and higher garrison bonus counters both unrest and culture penalties. By the time those bonuses are gone, unrest and culture penalties are mostly gone or completely gone.