Lateral quickness and defensive IQ are far more important to a switching defense than wingspan. CP3's wingspan is 6'4", four inches shorter than Westbrook's or EGo's, but he was a ballhawk and could guard nearly everyone. Capela's wingspan is 7'5" and he was clearly the weak link in the switching scheme. Marquesse Chriss' wingspan is 7'1" and he can't defend at all.
That was already acknowledged earlier in thread. Wingspan is the X factor for defensive minded players.
Harden wingspan is 6'11 vs. 6'11.75 (PJ). And they are both 6'5 barefoot. Average man wingspan is 2.5 inches more than height.