Maybe there is a parasitologist on this thread: the wifey has gotten involved with a cat rescue organization as a foster parent, which means I am now also a foster parent. We have adopted one of our foster kittens and are in the process of adopting a second kitten. Both have had the standard package of vet care including tests for various pathogenic organisms. They have been 100% indoor cats since they were rescued and will remain that way. We have given each them a single dose of Revolution Plus as ordered. The vet clinic used by the rescue organization recommends monthly treatments of Revolution Plus for all cats. The rescue organization's position is that it pays for one does of Revolution Plus then it's up to whoever adopts a cat to decide what to do, but it's not something they recommend.
For a possibly unrepresentative comparison, I had two indoor cats for 17 years, never gave them Revolution Plus or any other similar medication, and they never got fleas, ticks, or worms.
So is this monthly dosing cost effective? What's the number needed to treat?