Discerning whether abilities or extra power and toughness are worth more is non-trivial, quite context-dependent, and likely will be the key to properly building decks based around this new take on the mechanic, as there are ways to use it to add more power to the battlefield
Lots of enchantments both granting abilities like flying, trample, and growth as well as stopping your opponents from doing much of anything on the battlefield
In fact, it was the interest in the Mistform creatures that started the tribal boll rolling
As for Whispersteel Dagger, the card requires you to have an unblockable creature, plenty of cards to select from and mana not being spent on other more important threats
Just remember, you can only play one supporter and one stadium card per turn
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