I worked as a phlebotomist in a large family practice. I never hesitated to use the butterfly when 1) it was a pediatric patient 2) the veins were very small 3) the veins were roly poly and 4) it was a someone who was about to pass out at the site of a needle. Anytime my patients requested a butterfly I was happy to oblige. I don't know why other phlebotomists would not accommodate their patient's request.