StS has a number of enemies that shuffle bad cards into your deck, so this dillution works both ways. On higher difficulties more of these cards get shuffled into your deck. Trying a "thin" strategy with no countermeasures in such a fight (high damage output to end the fight fast?) can get you railroaded. You also start out with 10 starters that you want to dillute because virtually any card is better than them. Cards with "draw" and "exhaust" written on them also let you workaround the problem of drawing starter or negative effect cards. Picking up one or two versatile 2-cost cards early (attack or block card) can give you a good way to use your energy even if you draw mostly starters or negative effect cards while avoiding excessive deck dillution later on.
StS does offer you worse cards at the beginning of the game, and limited ways to remove or upgrade these cards, so picking up too many cards can burden you later on.
Really to be good at StS deckbuilding mostly requires you to know which enemies will come up when and building your deck sufficiently to survive them without shooting yourself in the foot long term too much. Jorbs is usually focused on building his deck to beat the hardest fight in the game while absorbing the minimum deadweight to survive until that point. Watching a stream can give you a rough idea of what a good "pace" for adding new cards into your deck is.
Jorbs can reliably reach the heart with a much more challenging deck than you or I can. He's also playing a game mode that imposes challenges that don't exist for new players. Just shoving a lot of good cards in your deck might be pretty good advice for new players up until about ascension ten.
If you're in Act 2 and headed for an elite, you have to think, "can my deck beat book of stabbing? gremlin leader? taskmaster?".
If right now your deck sucks against the book, and you're offered a disarm or caltrops, it might be worth picking it even if it's doesn't generally make your deck stronger. Cause otherwise you might get stabbed to death.
On the flip side, Demon Form is a very strong card, but it's practically a curse for most of Act 1, because it's too costly and slow. So sometimes you might skip even a strong card if it doesn't help you right now.
I'd say that's probably the most important "ah-ha" moment where my winrate really started to significantly improve. That and knowing when to min/max # of combats while pathfinding.
No, not necessarily. But my brain wants to pick one anyway.
At high difficulty levels you need to pick a card (and possibly buy cards at shops) through most of Acts 1 and 2 because you won't be able to reach the boss without a bunch of damage commons.
StS lets you not take a card when you are offered one, and often this is the best play, particularly as you go later into the game and have a deck that is stacked with powerful cards.
Fundamentally having more cards is bad, because it increases variance in what you can draw. Obviously taking a good card can be worth that cost, but often players fall into the trap of assuming the best card of the options given is worth taking, when really the question should be "does this card beat the average card in my deck, or does it dilute the power of the deck", because you are reducing the chance to draw one of the other cards in your deck.
Obviously there are edge cases: sometimes something may reduce the power of your deck right now, but introduces the chance for later synergy, which may be worth the cost—especially early on, but in general it is worth considering not taking the card.
The ideal deck is always the minimum number of cards that allow you to pull off your win state, everything else is just stopping you getting there (this is also why cards that allow you to draw cheaply are good, and one of the reasons why removing cards from your deck is powerful).