Obviously it's a good thing if an employee gets what he's worth, and it's nice if you can actually accurately determine that. But what's his previous salary go

Obviously it's a good thing if an employee gets what he's worth, and it's nice if you can actually accurately determine that. But what's his previous salary got to do with what he's actually worth, now? If you tell a potential employer what you're currently getting, doesn't it set an upper limit on how much you can get from the new job? If no one knows what you're getting now, you could get a 50% 'raise' when changing jobs, and still happen to be worth just that. Is there any way that a potential employer knowing your current salary can work to your advantage as an employee? I seriously doubt it.