Actually I think the reason that Amprollium is used so extensively is because of high death losses to coccidia. I personally have not had a problem yet, but if you read the forum it seems that people fall into one of two groups- the "I've never had a problem" or "I lost a TON of chicks to this". On my last group, I used 1 bag of medicated chick starter and they switched to a non-medicated grower. My feed store did not have any of the medicated this time so I am trying without, and will attempt to use good hygiene to reduce the coccidial load in the environment. Hopefully, I won't regret this!
Amprollium works as a coccidio-stat, which means it inhibits the protozoan's ability to replicate and populate the intestine by blocking thiamine metabolism. By preventing the coccidia from overgrowing, it allows the chickens to build immunity over time. Its a very similar concept to coccidia vaccination- by giving a small, controlled dose, it allows the chickens to build immunity to the parasite.
Incidentally, this is why you have to dose Amprollium for several days in all species. Other medications, such as ponazuril, actually kill the coccidia, but as far as I know are not used in poultry production due to cost. Likewise, by reducing intestinal nutrient loss, it does "boost" growth, or rather prevents the coccidia from eating your chicks from the inside out! Its much easier to gain weight when you're not spewing out blood diarrhea and becoming dehydrated. Broiler farms have a much larger problem with this due to lack of hygiene and environmental contamination.
In the EU, which has much higher standards for antimicrobial use than the US, amprolium has a ZERO day withdrawl time for both meat and eggs. On the FDA website I could only find a withdrawal time listed as zero days, but it did not specify meat vs. eggs.
The bottom line is, the beauty of raising your own food is that you don't have to eat anything you're not comfortable with!