While giving the n/s vent a clean just now it popped into the wing and thought I'd have a poke around to see if air could actually reach the area... as you can see there was (head) light at the end of the tunnel!
I think you'd have to be moving at quite some speed to have a decent amount of air flow but the n/s vent is certainly not only for show. As you can see the cavity is quite spacious and the area around the vent had a few fully formed leaves gathering.
p.s the clips on the outer edge of the vent that attach to the round metal clips on the wing are very fragile.