Webmin is a great tool, if you know what you're doing. It's a handy way to manage one, or many linux boxes without having to know how to format and where each config file is. It's a powerful tool for system administration.
Like any tool, if you don't know how to use it, you're going to muck things up, and because it's meant for server administration, you can muck things up badly, quickly.
With that said, when used correctly, webmin can reduce the time you spend doing redundant tasks, and can help you quickly find and change a configuration. I manage 20-30 linux servers and VM's, I understand the command line, the configuration files, and how to manage a server. I have webmin installed on all our linux servers, (no gui on any of them).
It's not reasonable to bash a tool because someone isn't using it correctly, or simply doesn't know how to use it. It is valuable to understand basic networking before conquering DNS, but it's commendable that people are trying, asking, researching, and correcting their errors.
Isn't that how we all learned this stuff.