I also look at it as risk:reward
I have about $100 invested in QT systems. I have hundreds, if not thousands worth of fish in my display. The last thing I want to do is have a $50 fish take out all of my livestock.
Exactly. He has $0 in fish currently and is buying a pair of clownfish. There is no risk to his current fish, since there are none.
For him, in the specific situation asked about in the OP, I would personally skip it, but if there were fish worth protecting, then totally quarantine.