6'5" is huge man. If he runs anything approaching a 4.5 in the 40 then that guy should be a WR, period...unless he has abnormally bad hands.
I am thinking that Saban is wondering whether or not he can get a 6'5" defender into position in this defense to pluck balls that were thrown a little low out of the sky. At the very least, Walls would be a good specialized role player for when we face any 6'4" or 6'5" receivers out there. We do play Randy Moss in about a month. I think McCareins kind of hurt us a little bit with his size even though he's only 6'2", and Drew Bennett stands tall at 6'5". Also, Cleveland's got a tall set of receivers in Braylon Edwards (6'3"), Antonio Bryant (6'2"), and Frisman Jackson (6'2").
I also have to think that if Walls doesn't have ideal speed for CB, he could play a little safety if he's any good at tackling. Not that Schulters should fear for his job, but Walls seems like a pretty classic role player type to me.
I was looking at Joey Thomas and I wonder if he's any good at special teams.