Mockups can have a bad reputation because developers hate design and people tend to use them mostly for design approaches. Look at who all speak out against them — developers.
Mockups are very very helpful to the client when you get into trying to explain why you chose certain methods to approach an efficient UX and UI. For example, why using Droid Sans on mobile would be a better approach over Open Sans. Sure, you can use code to show examples of the functionality but clients also want detailed information on why X rather than Y. Doing that and more in the browser? No, thank you.
there’s nothing wrong with a mockup, as long as it’s not “the website.”
Yeah, I agree. Definitely not talking about a full-fledged website. As stated above, I’m talking about mocking up UX functionality and UI elements and why doing or choosing X is better than Y.