I have a co-branded card but I would pick up the Mehretu design, were it to become available.

Yes, neither of the two designs have universally been "winners" (they're fairly polarizing) but I like the pop of style that it adds to the card. Certainly polarizing art is usually the kind that brings the most attention (both positive and negative)

To your point about usage, for me, the Platinum only ever comes out into the world when something cool is happening. Am I buying something expensive that needs the extra reach of Purchase Protection? Then the Platinum is coming out of the sock drawer. More often though, the Platinum is used for lounge entry (yes I know the app is usually a perfectly fine substitute). While I am not trying to "flex" on a cashier, it is just a little bit of fun (much like any piece of art) the brightens the occasion.