Yes, the main driver. Keeping python in resource files that are cleanly identifiable as python by external tool in source code control is part of that motivation.
Yes.
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What do you mean by "all the way out" ? Library scripts are easily floated to be accessible while working on UI components and all kinds of gateway resources. Total win. And even if not floated, the script editors help you pick the function you need.
Oy! The designer's plethora of event resource dialogs are not "nice", IMNSHO.
And further, the business/presentation boundary, in a well-organized gateway, is managed with inheritance, like the hierarchy I recommend over here:
Well tested business logic, that is not context-specific, belongs in that "universe" project. Shared presentation logic belongs in "ui_common", "perspective_common", or "vision_common". Project-specific presentation logic stays at the leaf, but in a project library script, to cover its possible future promotion to a shared resource.