Height surely isn't everything we're looking for in terms of evaluating a player's defensive ability. But wingspan surely isn't either. What about lateral quickness? vertical reach?
FYI, Trey Burke, who happens to be a nightmare defender, has a 6' 5.5' wingspan. But that doesn't make him capable of guarding anyone in the league
Also did you just repeated what I said? "Allen is bout an inch shy of league average for 2's". So he holds no size advantage over guards in the league.
Allen surely has some notable strengths. Unfortunately I don't think size is one of them. Largely cuz he's only 6'3, as opposed to the reported 6'5.