If you're going down the path that Maiar don't need to eat, then I have to ask why did Gandalf eat?
He could have just been trying to appear more as a man. You know, it would be pretty weird knowing someone who never took some food. But it also comes to mind that he really didn't have to. As
burrahobbit mentioned, it may depend on their level of incarnation. It would make more sense to me that since Gandalf was in a man's body he would have to take food to keep it running.
Also, I was kinda thinking photosynthesis for the Ents, because what would they eat? Meat doesn't make sense, and they wouldn't want to eat plants because they are almost a plant themselves. So they make their own energy with light and water.