I agree that there is better products on the market than WD that are specifically designed for gun cleaning. In my opinion WD is really good at removing water and protecting against rust, but doesn't work well has a lubricant. For gun parts that need a light lubrication I also carry a small bottle of gun lube.
But, the availablility of WD (at least in the states) is a huge selling point for me. Especially when I need quantities larger than just a couple squirts. Almost every garage, workshop, boat, or tool box has a can. And it's sold in every hardware store, chain store, or mom & pop convience store and gas station. Even our local grocery store carries the stuff in their limited mechanical section. When duck hunting in saltwater and bad weather I have used a three ounce spray can in a couple days.