The effect for this card is pretty good, but the cost and rarity isn't that great. I know I don't really talk about the rarity of cards all that much, but sometimes it does effect how I see a card.
For example, in a Zendikar block draft, you could be passed a pack of Worldwake with a Razor Boomerang in it or one with a Cunning Sparkmage. Assuming that you haven't really decided what you're going for yet, which one are you more likely to choose? Now I realize that not all cards can be made to have the same power level (not every planeswalker is going to be Jace, the Mind Sculptor or Tibalt, the Fiend-Blooded) but there should be some consistency in rarity levels. The fact that WotC designs cards mainly for Limited environments such as drafts or sealed events isn't something that should be ignored, but it's also not something that should be used as an excuse either. Having a Burning Oil on a stick is pretty good in a Limited environment, but is it really as powerful as Back to Basics, Citanul Flute, Gaea's Cradle, Morphling, or even Shivan Gorge? Not for 6 mana on a 3/3 creature.
It's definitely not the worst example of "why in Urborg is this a rare?!", but it's just not stellar enough to warrant its rank. Not a terrible card, just not a good rare.
Pros: Repeatable Burning Oil
Cons: Costs 6, only a 3/3