As far as calculating the expected value of foils yourself, it's exactly the same as calculating the expected value of non-foils, except they are worth significantly more and you get significantly fewer per box
Its hard to imagine a scenario where youd feel okay discarding three precious cards, so even if you arent brought down to an empty hand, Delinquent can be devastating if it makes you discard vital energies, Pokemon, and Trainers
It does a great job of shortening the length between a deck idea, constructing that deck, and testing that deck idea in a scenario
How many players look forward to every expansion for shiny new cards to tweak their deck
If you want to pull a card out of your binder or ask a friend to borrow their bomb high-rarity card for a couple minutes, playing a wish effect would let you use those cards
This has made decking the table hard to do: you need to realistically be able to mill out everybody in ten turns