Well, coins just suck, but green shells and bananas are useful for protection. If you have a coin, don't use it immediately. I'm pretty sure that if you're still holding it by the time you get a new item box, that new item is guaranteed not to be a coin; if you did use the coin, then you have a chance of getting a new coin. Also, holding on to that coin means you have protection against boos. Nothing sucks more than having a super horn or a mushroom in first and then having it stolen.
