While I like the suggestion in principle, I'm not sure how it can be executed well.
Issue arise is case duplication and sub-orderings. When you're viewing cases via the flat view, the case list is pretty directly analogous to a spreadsheet. When you're viewing cases as an outline, this analogy can break.
Exporting the flat view is always safe. Exporting as outline, in many circumstances, won't play friendly with a spreadsheet program: you'll get items duplicated, items you don't want.... The desired result for a user retreiving an outline is unclear to me. While we could certainly invent a technique of exporting outlines to spreadsheets, I feel like anything we could do would be confusing to most users.
Here's an a (very constructed) example of something that's fine as a list:
but nasty very un-spreadsheet-like as an outline:
FogBugz knows how to handle the outline-- spreadsheet programs would not.
