In a Killian deck, which rewards you every time an Aura enters attached to a creature, it pulls double duty, protecting your commander while triggering Killians ability at instant speed
Free mana engines should never be overlooked no matter how many dials are on them to slow them down
With their rising popularity and value, however, comes a significant challenge: the prevalence of counterfeit cards
This limitation was lowered a bit by Heartstone, and basically removed once Training Grounds appeared, which can reduce almost all of the untap cards' ability costs to 1 or 0, if you find a way to tap them for mana, thus allowing such things as infinite 1/1 tokens, infinite mana, or a +infinity power boost to all of your creatures
Maybe not choice #1, but if you want to play off the top, the original can get you there