Clarity is useful, and even necessary for instructional design, important notices, and daily conveniences.
Mystery is useful for getting the viewer’s attention, or sparking their curiosity, in the case of the cover design that Chip did for Haruki Murakami’s Colorless Tsukuru Tazaki.
Including just the right amount of information in a design so that the viewer can recognize what it is but still gets credit for figuring it out is a good technique, as shown in the design for the Diet Coke can.
I personally thought the Coke ad was funny. I would have kept it up, but I see how some might think it was done in bad taste.