You run mystic if your deck needs healing, you run fist if you have lots of low cost creatures to use it with…pretty rare for rust to be better then one of those, but if you are a deck with a horrible match up agaisnt artifacts then you want three. In all cases you either want all or none of those. That does not make your deck predictable, it makes it tuned. None of those your opponent will play deifferently wether you run one, none, or all.
If you need somethig before turn 5 but not after then you want three so you can actually get it before turn 5 consistently, even if you only use one. Otherwise it just turns into a dead card even more past turn 5 since you can’t reliably get it before then. The replace mechanic prevents it from being dead if you get it late, but only if you run a playset can you reliably replace to get it early.
Yea most decks need as much of X as possible, that’s what makes them strong. You can pick strong utility cards, but that does not mean you water down your consistency to do so. If you need a particular utility then you want more then just one of, or its just pure blind luck that you get it when you need it.
Pretty much the only time to not run a playset are with a couple things you will basicly never cast more then once in a game, stuff like Rite of the Undervault, Bounded Lifeforce, dominate will, or other high mana cards. Otherwise not running play sets actually makes you less versatile, less prepared, and less powerful. Because if you need something you can’t reliably find it.
If you need a certain tech enough to warrant a slot, usualy that means run a playset. Now there are games where running one copy is great, but those are games where you run play sets of tutors so you can find your one of utility cards. So perhaps if your trying to manipulate Artifact hunter, or Cryogensis this has merrit, but for the most part duelyst does not have tutors, and the ones it does have are random.