It helps to drill in basic syntax and increase your speed of thinking in the language. Also gives you an opportunity to practice debugging and optimization on a low complexity system, so you have all the concepts in your head when moving to a larger system.

It's really bad for things like project structure and tackling multiple complex, related problems. Bigger picture ideas get lost because of the scope of the problems.

Really depends on what you're trying to accomplish. Think of them as a set of flashcards. I'd say it's one of the better ways to bridge the gap between beginner programming books and actually building small, useful programs.