When faced with a player who sometimes does and sometimes does not wear a sweatband, if you want to determine if/how that factor will affect performance, there is some simple logic to apply:
If the player expects to sweat a lot -- i.e., work hard -- he will wear the sweatband.
If the player expects to stay pretty dry -- i.e., phone it in -- he will not wear it.
Please note: this logic only started being... um... logical... after the NBA ruled in 2010 that players may not wear their headbands with the logo upside-down. Prior to that rule, guys would wear the band just cuz it looked cool, thereby obviating the logic of it all. Now they wear it to soak sweat.
If you don't believe me, look it up. It's all there in the stat book. I swear. :\