distalizing a molar is not an easy process. Also losing space in the molar area can cause the premolars to be blocked out and have nowhere to erupt. this causes the expense of orthodontics to increase and the length of orthodontics to increase.

Yes sometimes we do extract teeth in a process called eruption guidance. this is usually canines to help the lower anterior crowding to unravel. this is always done with the parents understanding that Orthodontics will need to be done in the future. we are stealing space from the canine area to allow for the front 4 to unravel.
Orthodontists will sometimes want to do serial extractions because the arch is too small this can start with primary teeth coming out early to try and allow certain permanent teeth to erupt early so that they can be extracted to give more room for the rest of the teeth. Orthodontist are very careful about this type of case because there can be other aspects of facial esthetics that can be effected by removing permanent teeth.

Anyway there are teeth that do need to be extracted in crowded mouths but it is never the Primary Second molar. Space loss in the primary second Molar area causes all kinds of Orthodontic problems. So yes it is better to keep primary teeth than it is to extract them in most cases.