Just how dangeorus is spyware?

It's something I've never quite understood, nor am I understanding even more just looking for more information on Google, as it's hard to tell whether it's just marketing hype or real facts. Just HOW dangerous is spyware, besides the usual loading up porn dialers, cookies, and fucking up the registry? Basically it's just a danger to the OS's stability, and not to the user's actual data, like, say, passwords for important stuff like banking and Paypal?