Yeah, I understand the question: Why? Honestly I'm not sure...something doesn't feel right about giving them medicine when nothing is wrong. If I started to detect an issue I'd switch immediately without a problem (hence my question about early warning signs). I suppose it's similar to how I raise my kids: I only give them medicine if it's really necessary, and not until then.
I'm very close to just switching, but it doesn't feel quite right. I'm still on the fence and it's a perfectly legit question.
Aprolium blocks absorption of some vitamins which can lead to vitamin deficiencies, but I didn't learn that until after I decided not to use meds. I just really don't like the idea of feeding my chickens (which I will later eat) anything I wouldn't be willing to eat myself. Most people don't mind because they are only on it for a few weeks, and say it doesn't stay in their system for long enough to matter. I don't know how long it takes for aprolium to leave a chicken's system, but I imagine it's longer than most people think. Did you know it takes a week for one bottles worth of formula to leave a babies system? I would much rather feed my chickens medicated feed than have them all die, but I want to learn how to avoid it.