QtQuick doesn't need to be compiled. That's just an optimization. I've never used it or needed it, though admittedly I haven't worked on massive QtQuick projects.

I think the main reason that's a commercial offering is that developers of proprietary software like shipping binaries instead of human-readable QML files.

The QtQuick compiler is only needed if you want to optimize startup performance (by compiling assets to binary form in the build process). I've never needed it. I'm guessing you've never actually tried QtQuick based on your comment.

Not since 5.11.

You can use the Qt Quick without compiling though it slows the loading.