Whether or not interns are useful depends on your business and the task.
If there is a lot of knowledge needed that they don't have, they are a drain. They will use up more resources getting trained and getting up to speed than they will be able to contribute. This has been my experience where I work, in our departments of software development, engineering, biology and chemistry, and production. The tasks have too much project-specific knowledge required.
If they already have the knowledge and can complete the task, then it can be useful. This has worked out in our legal department once, for example, but only because we had a well-defined task that a law student could do and had one available (so that we didn't have to screen a bunch of them, which takes time). It might be useful in a marketing or sales department where you want someone to do a fairly simple task that doesn't take a lot of explaining (like a web search for competing products or looking up contact info on all companies that meet a specific set of criteria) or general office work if the task doesn't need much training. Still, you have to figure in the time to locate and screen applicants.
So, it depends, but usually software development is not a place where an intern will save you money -- at least not in my experience. Even things that volunteers do in AH (CM'ing, making terrains, making skins, for example) are things that an intern would not be able to do.
