I've noticed the same - I've had to submit zeros (although not recommended, was the only way). Interesting enough, if you do SetDataCell in a formula within the application and set the same property as True, it will clear the data.
For strictly offline data collection, CSV loading, Form loading, SetCell are the main ones. But with the new spreadsheet functionality, couldn't they just collect inside?
I think it depends on what you are doing - if you had a Form that the user would want to submit in Excel... previously, they would either need to export it from the Forms workflow channel or log into Excel and refresh it. With spreadsheet, you can just build the Form into Excel and put it right on the Forms workflow... the user doesn't need an Add-In as well which sometimes is the issue.
And SetCells in and of themselves are ""dangerous"" ... I typically try to avoid if possible
