Kind of cool, but also kind of concerning. I'd rather that they flip it so that the single cell answer is the default and the multi cell one is the exception with the @. That way would be better for experienced users who know what they want and how to structure the data. Their proposal is better for casual users who just put in formulas without knowing what they're going to get.
It will encourage sloppy, spread out spreadsheet design to accommodate all of the arrays that will be returned by these formulas.
Also I'm sure that my clients will have a bunch of messed up spreadsheets where the user thought they were getting a 1 cell answer, but they overwrote part of their model. OR where an array extends past where the range they set in a formula somewhere else, and the data gets excluded.
Microsoft