You cant see always in naked eye! You should use microscope, as i do, you should do fecal tests as I do, believe me your chickens have worms! And they also have Coccidia, and they have parasite called Flagellate, and I am taking a risk here they also have the MDV that causes Marek disease, and they do have worms!
The issue is, was and always will be equilibrium! If you have happy, well fed, well provide chicken, that dont have a lot of stress causing reason you chicken will do fine! But if the stress load is high, like in winter time, the stress hormones, like Cortisol start to kick in, and the first thing the do is to suppress the immune system and in a blink of an eye you will fined yourself with a big problem! Problem that you could avoid by making some preventive steps, like worming your chickens!
And I don't see any connection between a dewormer that kills wormes that are Eukaryotes and the microflora of the chicken that is Prokaryotes!