I would have been impressed if they had bumped hips/butts like dancing 20 or 30 years ago. Bumping knuckles is rather classless. I think most people are unimpressed with knuckle bumping.
its something younger people do ... it would be the same if Obama walked up to a podium and started saying, "yo yo.. what up dog?" it just looks and sounds stupid.
it was fairly unbecoming for a wife to do it to her husband .. they looked like teammates at a pick up game, not presidential.