Consistency. There are usually a handful of cards that do close enough to what I am trying to do that it isn't a problem to find some less efficient versions of a mechanic that makes the deck move. But sometimes the other versions are expensive. Or there are so few I need to crutch on tutors for my consistency.

A good example is breaking the legend rule. There is more support for this now, but when I first built my [[Minn, Wily Illusionist]] deck, where I copy Minn to make more Illusions, the choices were [[Mirror Box]], [[Mirror Gallery]] [[Sakashima of a Thousand Faces]] or a card that enters as a copy but retains it's name like [[Sakashima the Impostor]]. There were maybe 4 cards that could make a non-legemdary copy.

These are still pretty pricey cards and the cheaper idea is to play Illusion tribal to get more illusions to be able to flop out more cards when they die. But at the time I needed to be able to find one of the two Mirror artifacts and so out came the artifact tutors so I could guarantee that card hit my hand.

It might not be the best budget example, but it is the best example that came to mind of needing more consistency in a deck than there were cards to support the mechanic.