Alien invaders as a parasitic class

One thing we humans like to do when we are feeling uncharitable is to cast some group of people that we are NOT a part of in the role of a parasite class. It can be the parasitic rich people or the parasitic poor people or the parasitic telephone sanitizers. It's better to keep this as just talk because when you try to take action based on this viewpoint, you will end up like the old Soviet Union who essentially got rid of their parasitic class of telephone sanitizers and then died from a disease contracted from a dirty telephone.

But the idea of alien invaders as a parasitic class on the backs of the human race is quite a useful one. It certainly provides a possible motive for an alien race to go to the trouble and expense of conquering Earth.

Conquering aliens with an advanced enough technology to keep themselves in the saddle could live on Earth in vast numbers without having to trouble themselves with dirty jobs such as farming or factory work. Their entire workforce could consist of supervisors and military/police members--- higher status jobs, in Earth cultures at least.

Aliens desiring to create a slave-owning colony are a bit more logical than aliens here to ship food, mineral resources, or edible humans back home. The expense of the travel is likely to make such trade wholly unprofitable. But creating new colonies --- especially ones with a native population to do all the menial jobs--- is something that might be worth a great deal to an alien race.

Creating colonies might be something that alien races would do because it helps things at home--- the colonists and soldiers they ship off might be dissidents or potential dissidents, or the whole enterprise might prove stimulating to a stagnating culture.

In my (stalled) NaNo novel 'Bakoun', the aliens also have the motive that their current primary planet--- also a conquered world--- suffers from some problems. The natives of that world, after some thousand years of captivity, have rapidly falling birthrates, and are increasingly less efficient workers. In the experience of the Bakoun, this is an expected thing, and they have a cure--- conquering a fresh world. And the world that they have chosen is Earth.

While the aliens may plan to let the Earth folks do all the heavy lifting, one must be careful not to make them too much of a 'parasitic class'. In the real world, when one looks at things more closely, even a person who seemingly consumes resources without producing, such as a retired senior citizen, may be making highly valuable unsung contributions. Our future alien overlords, likewise, are going to be making some contributions to our society whether they want to or not.

Alien invaders might introduce beneficial advances in technology and medicine. They may end warfare and tribal fighting among humans, if only by giving humans a more tempting target than one another.

In addition, aliens may be more beneficial for some Earth cultures than others. In Harry Turtledove's Worldwar/Colonization series, the alien Race liberates the Jews of the Warsaw ghetto from the Nazis, becoming the Jews' liberators. The Race doesn't care much for the Nazis, however, and, among other things, nukes Berlin and other German cities just to show off their power. The result is that other humans reject the Jews for sucking up to the alien invaders, and make nice with the Nazis in the common cause of fighting the Race.

Question--- if you were writing an alien invasion story, in what way would the invaders be 'parasitic'? In what way would they be beneficial? Which human groups would suffer more under alien rule, and which ones might benefit?