Seems like a huge waste of money to me.
Not really. If you can defend a billions dollar carrier battle group from a missile while it's still a hundred miles out with a round worth 25 grand it'd all be worth it. Their use is somewhat limited by the curvature of the earth though, meaning it's only really going to be useful against incoming missiles or aircraft. Something that fast would have a fairly flat trajectory, so basically the horizon is your maximum range.
I vaguely remember reading something about Reagan's Star Wars plans, and how one of the rail guns uses was envisaged to be shooting down enemy satellites. Now that would be pretty awesome.