I used to say the same thing as the OP. Bought my fish from a great LFS, with knowledgeable staff and nice stock. I'd been watching a group of taeniatus fry for over a month there, before purchasing a pair. They died suddenly within 3 days, and then I started losing my other fish. Went back to the store, and the taeniatus were almost all dead, and they were losing fish in their other tanks also.
It's still not totally clear what parasite was introduced to my tank by those fish. They had wild parents, and the breeder wound up losing all of his stock also. It *seems* to be a really resistant strain of velvet. I've treated with multiple meds, and it goes away, but keeps coming back.
To date, because of that one time with not QT, I've lost a pair of breeding rams, a yo-yo loach, a glowlight tetra, two apistos, a platy, and a pair of nannacara. My female silver angel has recurring infections - she recovers on her own, without medication. My male kribensis (part of a breeding pair) is infected right now - was brought on by stress when he had a tiff with his mate (they had some "custodial" issues over the fry when I cleaned the tank while they had their babies out). I don't think he's going to make it.
It's quite costly to treat a 90g tank, especially when none of the meds seem to work. I've lost some really nice fish, and will probably lose more. I won't be adding any more fish until my tank has been without any sign of the disease for 3 months. What a DRAG. I've been battling this little bug for almost 6 months now.
All could've been prevented with QT. It was sooooo not worth it to skip QT
