Obviously the ability to cast a card that costs three or less for free is a major plus, but if Yahennis Expertise ends up being an unreliable or too-slow sweeper, then it wont be played enough to warrant this slot
It naturally ramps up in loyalty very fast, and it has a defensive ability to create a growing 2/2 Spirit token
Even though any one of these factors can contribute to the overall design of a card, a card is best when all factors come together
shes meant to just chill out while everyone gets to do stuff for free
And beyond this effect, a 2/3 evasive for three is a very solid statline, meaning Wildcat can do some work even in matchups where his dismantling expertise isnt required
Creatures and planeswalkers effectively cast just their colored mana, and you activate Tezzerets +2 ability for a huge pile of damage and lifegain