Smith is not a bad help and team defender. He, like Sammy, lacks the bulk to be an effective one-on-one defender down low, and the T-Rex arms don't help him when he defends on the perimeter. He has a year's head-start on Speights in understanding team defense at the NBA level. I think we will see Brand and Reggie guarding the heavier bigs with Smith and Sammy guarding the guys more their size and providing weakside help. Speights should eventually start eating up those minutes and he has the talent to be a good defender if he puts his heart and mind to it, but as a rookie who was not a defensive specialist in college I would expect a learning curve on D for him to start the season. Thad was eventually a very good defender last year, but he needed to get his game up to NBA speed and there is no reason to think Speights will be any different.
As far as grossly overvaluing Smith, based on what? The idea that a mobile young big man who can shoot and block shots has some value is wrong. He is mobile, but not superquick. He has good size, but he's not very powerful, although calling him flabby constantly is a complete mischaracterization. He has good potential as a jump shooter and gets out on the break very well for a (near) seven footer. He also has limitations that will keep him a bench player for his career. He is a willing rebounder and has a good attitude ane some mental toughness that he displayed last year. Considering his modest salary at this point and his strengths and weaknesses he is a valuable and economic role player.
If you want to talk about overvaluing someone, let's talk Marcus Williams. The guy had major red flags coming out of college, has done nothing in the pro's for a team that took a chance on him and who needed him to come through at least as a strong back-up PG. The Nets have a PG in Harris who is more of a combo-guard, yet Williams is still in the dog-house although they need players at his position. Yet you want to give up smith and a first rounder for a guy who probably won't have his option picked up next year if he deson't crank his game up this year. I would absolutely take a chance at williams on the cheap, he has good skills and potential, but giving up essentially two first rounders for him is major overvaluation.