If you are very clear with your strategy and tactics and just want someone to execute, hiring an intern is a good option when you don't have enough capacity to implement everything you have in your mind.
But if you just want a magical intern to fix your marketing problems without clear guidance and detailed expectations... that's a different story.
Yes, it was a terrible experience. I would strongly advise against hiring inexperienced marketing employees. It might work for developers, designers, and managers, but it's not a good idea for sales or marketing managers.