All cards with entwine, when cast, give you a choice of two different things to do. One Dozen Eyes lets you either put a 5/5 token into play, or put five 1/1 tokens into play. It's good on its own, but it has entwine for when you have lots of mana. If you play an additional three green mana, you get to do both abilities, so you get a 5/5 and five 1/1s! Entwine also comes in hand with cards like Promise of Power. You get to draw five cards and then entwine to get a demon in play with P/T equal to the number of cards in your hand, which will be at least 5, since you get to draw five cards.
Entwine is certainly, at many times, a very useful part of choice cards. Although entwine can be expensive, it can also be worth the price.