Even though the excerpt is just an introduction to screenwriting basics, it breaks down the most important and fundamental elements of the craft. Not nailing down your story's structure is like dropping your audience in the middle of nowhere without a map and expecting them to make it all the way home. It can be done, but 1.) it probably won't, and 2.) if it is, your audience will be super pissed when they get there.

Now, there are no hard and fast rules when it comes to any part of screenwriting; if you don't like the monomyth, you don't have to follow it. Not all monomythic scripts are good and not all good scripts are monomythic. For some, the Hero's Journey template is the answer and works like a charm, but for others it may be Syd Field's plot points paradigm or Robert McKee's story beats that gets it done. Maybe you don't like the three act structure and you want to go with five, six, eight, or nine.  

The most important thing is that you provide your audience with a clear roadmap of your story so they always know where they are and can navigate from beginning to end