For starters, having to exile two cards is a very steep cost, and the result isnt that devastating
If you have creatures like Goldspan Dragon or care about artifacts (such as branching into UG or RG) then Provisioner is a slam dunk include
Control then becomes too good, because it has strong tempo AND card advantage
Colorless mana costs for a card can only be met with sources that produce colorless mana, and cannot be met with any other colored mana source, compared to "generic" mana costs which can use any colored or colorless mana source
If they dont have removal its effectively card advantage, and you also get information to plan your next steps
That said, your success with a given deck comes down to how its constructed and how you play it