Wemby's not a natural rebounder, his first instinct is clearly to leak when he's defending on the perimeter and a shot goes up. Seemed to do it every single time in this game. You can kind of understand that he likes to have a "head start" - he's so incredibly tall it's affecting his straight line speed a bit (sure he's incredibly athletic for his size, but you can't fool physics), and leaking removes the need to do so many hard accelerations per game on fast breaks. When you're that gigantic, it all adds up.
But yeah... deciding what position he's going to defend is going to be a problem for the Spurs when the real games start. It'll also be interesting to see what kinds of strategies other teams utilize to neutralize his defensive impact. I presume everyone will try to drag him outside - particularly as it's apparent that he's not that likely to venture back into the paint to challenge shots from the weak side, as said above.