This is a completely personal decision, neither one being right or wrong. If there was a disease in my area that was rampant, then I may change my mind about vaccinating for that particular disease. And there are many diseases that have vaccinations available besides Mareks or cocci. To some, it would be cost- prohibitive to vaccinate for every possible disease a chicken may encounter and some vaccines do automatically make the bird a carrier, if I have read the information correctly. So, it's up to each person to make his/her own informed decision.