I think there are three different levels here:
The situation of the writer, the situation of the reader, and the situation of the viewer.
I believe that the 3-act or 8 sequence or 22 sequence or whatever is a kind of notepad for writers to help keep some sort of momentum and engagement for the audience (reader or viewer). As long as writers tell compelling stories, it doesn't matter what the numerical systematization is.
Readers of spec scripts are a strange hybrid. Their goal is to discover stories that will make a profit at the box office, so they're audience-oriented, but there is no real movie or real audience for a spec script, so readers need some sort of guidance (other than Tarot cards or a Magic 8 Ball). So they tend to rely on formulas that structure gurus have created. But these theories are descriptive of what worked in the past, not prescriptive of what will work in the future. A further flaw of using conventional structure to evaluate a spec script is that every theory I'm aware of is too clumsy. I've been told that a not-very-bright music student can write the theme or subject of a fugue that is as interesting as the theme or subject of a Bach fugue, but when you create the fugue around the theme of the novice student, it falls far short of the engagement of a Bach fugue. There are tens, maybe hundreds of micro structures in a 3-part fugue, and they're far more important than the largest building blocks. It's not that a great script follows or avoids the three-act structure; it's that the engaging script is built on hundreds of smaller components (for which there is as far as I know no single grid or template that creates engagement).
Most viewers are unaware of structure (although it's not unusual for a reviewer or layperson to talk about act structure or character arcs). Viewers want to see something new (otherwise why wouldn't they just watch the same film over and over), but they need something familiar in order to undertake the perilous journey that is moviegoing. A 3-act (or 5-act or whatever) large-scale structure can provide them with the familiar, but how the writer handles the micro structures is likely to create a feeling of novelty and discovery for the audience.
