What authors usually do is is knock out a freeform treatment where the story is described in prose, which is then broken down into scenes. It is helpful to have the overview in mind during the writing process, but existing tools such as Final Draft provide a plethora of writing aids for that, from virtual index cards to graphical character timelines.
Bear in mind that when you work with finished scripts like the Alien example, you've got a survivor bias problem, because almost any story that makes it through the screen has been through multiple drafts and hundreds or thousands of mini-edits. So while it seems very natural to lay that out in a neat hierarchy, that doesn't really reflect the writing environment, which is a lot more messy.
As for keeping track of story context, this is job one for the writer - you can beef up the dialog or whatever later, but writers, directors and other keys need to be able to keep the entire story in their head at once and know what the inputs and and outputs of any given scene are, not least because 99.9% of films are shot out of chronological sequence and so being able to keep a handle on that story context is essential for guiding the actors. Obviously you don't try to remember every last little thing at once, that's why we use storyboards and breakdown sheets, but you do need to be able to articulate the whole story off the cuff at the drop of a hat.
I do think this would very useful for film students doing analysis, but I can't really see myself writing a script in it, although I'll try doing some treatments with it.
BTW I also feel that if you offer people the opportunity to try it, you should let them try it, not switch to asking for a signup. That really annoyed me.
