See, this is where I think you're being uncharitable. Naturally different advice is given to people at different stages in their development, and what this magician is alleged to have said is not among the "dumbest" thing I ever heard.
I didn't say it was among the dumbest thing(s) I ever heard. I wrote that "among the dumbest advice I've heard coming from a professional magician". I've certainly heard a lot of things that are dumber- some even advice coming from self-proclaimed "professional magicians" who really aren't.
Different advice is also given to people with different goals. If your goal is to never perform except in situations where you can reasonably expect to borrow a deck, there will still be times when no one has a deck you can borrow. Some decks you'll borrow can't be used for anything that requires technical handling at all- decks that are 10 years old, dirty and dog-eared and twice as thick as a normal deck from all the liquids that have been spilled on it over the years. Limit yourself to only tricks that can be done with those decks and you'll have a limited repertoire.
Paul Cummins is best known for his From a Shuffled Deck in Use notes
Paul Cummins also does tricks that can't be done FASDIU. (Workingman's Wild Card is published in his FASDIU notes and it can't be done with a normal deck at all). And the fact that he's best known for FASDUI tricks doesn't mean he'd try to perform every trick he does using any crappy deck he's given. (Paul also does a lot of bar magic and I'm pretty sure he is rarely able to borrow a deck from the bar patrons.)
Swain (maybe you were the person quoting him) is quoted as saying a spectator should be allowed to shuffle and inspect the cards whenever she pleases, and each of those styles is consistent with that approach.
Perhaps true, but still not really pertinent. Swain does many tricks where the spectator couldn't shuffle the cards whenever they please, but he also knows how to handle situations to prevent that from appearing to be a problem.
Yes, yes, there is a significant difference, but it approaches the ideal of using something that's borrowed, which gets to the heart of magic. With magic the power is supposed to be in the performer, not the prop, so when you can play off some regular object completely impromptu, then the effect dramatically increases.
Still true, but it's not difficult to pass off a regular deck of cards as a regular deck of cards.
With magic a borrowed deck defeats one of the most common explanations.
All true. But none of it really addresses the point unless your point is that you should be able to perform every trick you do with a borrowed (crappy) deck. I'm fairly sure you've already agreed that's not the goal. But even then, there are times when if you alway practice with a crappy deck you're going to be in trouble if you borrow a new deck.
How does the other old saying go? Produce a sandwich, and you have a good trick. Somebody says they're hungry and THEN you produce a sandwich... you have a miracle.
So if someone says they feel like a game of poker and then you produce a deck of cards, you have a miracle. But if you have to first borrow a deck to produce you'll lose most of the miracle aspect.
People are suspicious of cards in the context of magic, and they're doubly suspicious of people who carry around their own pack of cards to do magic, especially if you're just some dude. And that's not unreasonable.
None of which addresses the point. My point was never that you shouldn't borrow cards when possible. My point clearly has been that you should have the ability to do magic with borrowed cards. But I maintain that learning, practicing and performing with cheap, crappy props (cards or otherwise) should not be your defining factors in learning and deciding what to perform. While I pretty much agree with most of what you wrote, none of it really addresses the fact that the advice given was bad unless you do agree with that and your only point is that it's not as bad as I said.
Some examples of reasonable advice might have been:
-Learn tricks that you can do with any borrowed deck because sometimes (often) you'll get a cheap crappy deck.
-Practice as many tricks (that you're going to perform) with different deck in various conditions so you can do them with most borrowed decks.
-Borrow the deck whenever you can.
And since the most common deck used (at least in the southeast US) are Bicycle Rider Backs, there's a good chance that's what you'll get when you borrow a deck of decks. While borrowed Bicycle decks can be in poor condition like any other brand, they are not "crappy quality" cards- and that's what DrugFreeAlcoholic was advised to use. So at least in large parts of the US, you're specifically not using the cards that you'd likely be able to borrow.