it's the only real way to diagnose what internal parasites you've got. (except for the time one of my discus pooped out a 4 inch tapeworm

)
i've seen someone anesthetize a fish, take a tiny gill sample and examine it under a microscope to look for gill flukes. (they were there and I got to look at them under the scope -evil little things with a spiked anchor on their tail tearing off pieces of gill with their mouths). the 'pros' don't guess, they look for it and treat what they can actually see - allows them to use the right medications and also know when it's time to stop medicating.
'arm deep' no - I think you're thinking of artificial insemination in cows.
Who's hungry now?
windsurfer