Green, rob and Durham is a very small lineup. Not to mention we have little guard depth.
Most teams play a 3 guard lineup these days. You think Smith is a better perimeter defender than Durham? What is Smith going to do offensively with TJD and Davis/Brunk in the game? Again there will be instances where Smith starts/plays a lot of minutes against bigger, more athletic teams, but Durham over Smith gives you a ton of more flexibility.