Nash is a very bad defender at the point of attack, but let's not overplay the negative effect that brings to the Suns. There are some starting PGs that may be worse defenders than Nash - Fisher, Calderon, Bibby - and several others who may be better but are still in the same tier - Curry, Harris in the last 2 seasons, Arroyo. Even guys like Jarret Jack or Baron Davis in lots of nights, Tony Parker, Udrih, Rodriguez... do they do a lot more for your defense than Nash?
Every year the Suns defense is just a little nothing better when Nash is off the court - more 0.8 points allowed per 100 possessions, more 2 points allowed per 100 possessions, that kind of difference - and he's actually playing against starters.