Actually, YES. Every time I have gone to a doctor, they do in fact want a blood or urine sample, often both. If I go to a doctor and tell him I have a stomach ache, he checks my white cell count, he doesn't just prescribe me something without looking for evidence of what's causing the problem. I do not think I would go to a doctor who treated an internal problem in that fashion!
"Anorexia" in snakes, 99% of the time, has a concrete cause. The first thing that should be done is to look for the cause. Once environment and prey preferences are addressed, it's time to look for signs of an illness. Parasitization and infection are two common causes. It's important to determine what's wrong so you can treat it properly and completely. I stand by that opinion.
These drugs are not harmless. They are toxic chemicals. Some individual animals have adverse reactions to them. It's rare, but it happens. In an animal already weakened by some underlying issue that has caused it to stop feeding for a year, it could do more harm than good if it is not the proper treatment.