I never used to quarantine new specimens. Hell, when I started 46 years ago, 'quarantine' wasn't even remotely used when aquaria were discussed.
As the science of fishkeeping evolved and progressed, quarantine methods came into being. Reason being, as I see it, for the purpose of reducing pathogens in an established system. A pathogen in a small rectangular box can run rampant and prove deadly in a much shorter time than when the pathogen is noted in the wild. Also, the "natural biofilter" that exists in the wild can't be found in captive situations. I'm not referring to the nitrobacter-based filtration systems used in the hobby. I'm referring to the naturally occurring organisms and chemicals that may keep pathogens in check in the wild.
I still don't quarantine every specimen. However, when introducing a new specimen to a tank with $10k of fish already living in it, I definitely DO quarantine first. I don't roll the dice on $10k in the idea that they'll make it if they're strong enough. I'm going to ensure that I provide the existing, and new, tankmates the best shot towards remaining healthy and active.