Crowder's complete inability to dribble is as glaring a weakness as Smart's shooting woes. Smart is a significantly better defender and team leader.
What the hell...?
Marcus Smart is one of the worst ball handling guards I have ever witnessed in my entire years of watching professional basketball. I legitimately think that Kelly Olynyk and Al Horford might have better handles then Smart.
The only thing Smart does well (handle wise) is use his wide frame to shield the ball from defenders - and that's not a skill, that's just a natural benefit he gains from being a very solid guy. Beyond that he rarely ever needs to handle the ball against pressure since he's such a non threat that opponents rarely ever apply defensive pressure on him, and every time they do he looks like he's about to bounce the ball directly off his foot.
Crowder isn't a good ball handler but it's not a critical skill for a SF...however Smart is an equally garbage ball handler which is a serious problem given he is a point guard.
As for leadership, what leadership? Smart is the basketball equivalent of an angry Orc with a big club. Just tell him where the bad guy is and he will charge in an Orc rage and club whatever you point him at to a pump. He's not exactly the smartest or most sophisticated basketball mind in the association.
Also Smart definitely is NOT a "significantly better defender" then Crowder. That's not confirmed by eye test and it's not confirmed by any statistic (basic or advanced) that I have ever seen. Because it simply isn't true.