Brook, velvet, and uronema have taken out their fair share of tanks.I agree with Nick that a clean fish will just pick up any existing controlled parasites in a given system. On the other hand, I feel that the QT process is still important to keep out the more deadly nasties that are not going to really be controllable like brook, velvet, uronema. In the confines of an aquarium even with strong UV these can wipe out a population quickly
I guess there is always going to be someone who advocates for "no quarantine" given that qt'd fish can always pick something up that's already in a customer's tank.
Personally I would pick quarantined fish 10 times out of 10 if given the option. If there is some latent infection in my tank, it's in some kind of asymptomatic balance and I'm less concerned about it.
Looking back, I see that Nick said that personally, he wouldn't offer it. Does he manage a LFS?