Hello! Thank you for trying Scripto out and posting this. You are correct it is a non-strict system -- the flags and branching examples are just ways you COULD go about structuring a script, with the processing work passed on to your engine. So as to why some might prefer this over a spreadsheet format, it comes down to a couple things:
1) Fluidity of writing. In a script, you can write a scene top-down and think about every line in context. In a spreadsheet, this context and flow can be lost when hopping between cells. It is also harder for voice actors to make sense of spreadsheets than the screenplays they are generally used to receiving.
2) Automatic per-line GUIDs. Our system assigns a unique ID to every line of every script on creation, and that ID travels over the API. This is a key part of localization and VO that is often assigned later in the development process, and can be labor-intensive, so it's time-saving to generate them in Scripto.
You're correct there are some affordances we could be making to global variable lists and auto-completion -- we're working on that! In the meantime, if you are looking for a fully interpreted language whose affordances are more strictly defined, I recommend trying out our implementation of the Ink format -- Ink is well-documented and has been used in many commercial projects. Ink will also compile in-browser and let you click through the choices you've written, and there are a number of downstream integrations that already exist for bringing it into Unity, Godot, etc.
Thanks again for your feedback here!