giving the user that much control, using his example of a spreadsheet on his computer, makes me cringe thinking about how much the user could mess things up with the data.
By contrast, it makes me excited thinking about the possible solutions to the problems you just described. What could be done? Maybe there is a revision system in the web app that shows the changes which must be confirmed before applying? This also would give you web app the ability to easily version control data.
Just like with the web application developers will just need to control what a user can do. Users can break native client apps by screwing with config files now but for the most part they don't I think as long as a developer creates some friction (like hidden files) it won't be too much of a problem.
