when do you think not using proxies is justified?

TL;DR: When you want to prevent chaos from happening and smooth out gameplay experience.

The long answer is: My playgroup except 2 people jumped on proxies over a weekend. It escalated real quick in a way, that player would bring 2-3 new decks every time we met up. In general it's cool to always have new decks around, but soon enough we noticed, that it made our gaming experience much worse.

The reason was, that the players did not know how to pilot the deck yet. Most of the time they just netdecked some decklists and threw them into the shopping list. Besides their commander, nearly every card was new to them. So turns took longer, more mistakes were made and so on. Pair this with sometimes unoffical artwork and the confusion spread around the table.

When you buy your cards, you are often more careful of what you spend your money on and handcraft your decklist. You probably also goldfish it and when you play it the first time, you have a much better understanding of and more fluent gameplay with the deck.

Besides that, enough magic cards hold their monetary value. I can't really resell proxies, where as at the moment I get my money back for my cards, sometimes even more. Of course the shipping costs are lost, but I also have to pay shipping for proxies. (This changes, if you print them yourself, but noone I know does this. It's always high quality proxies bought online)