The itch from just one flea bite can be maddening to both pets and humans. Been there - done that! Then of course there is the issue of how sick it can make some cats or dogs.
If they don't treat for the fleas, then their house will soon be overrun with them. Not a place that I would want to visit, that's for sure. And I can't believe that someone actually thinks that there are specific species of fleas - one for cats and one for dogs, and that the 2 don't mix. OMG... Fleas aren't fussy - cat/dog/human - as long as it has a warm body, then that's all that matters to them.