I cant say I have noticed it being poular on any of my non-EU-EU flights, in fact most times on board the cart just whizzes past and no one stops it.
However what I noticed when I flew to Rekyjavik on FI
a few years back, the whole plan was full of Icelandic people returining home, and EVERYONE of them bought something. It took ages! I remember the cart being in the aisle for a very long time.
But as someone else pointed out, it has become an emotional thing to do, more than an economical one. After all, when one starts a plane journey it is part of a holiday, and we all love some pre-flight shopping and goodies to buy to put us in even better spirits.