The NBA fan experience is a strange one. There are plenty who wish NBA fans shared the same passion you see at the college level. Very few basketball arenas have the atmosphere before the Playoffs of an engaged fan base.
The NBA is a business first and foremost. And there would seemingly be a smart correlation between attendance and winning. Teams that struggle to win are going to have a tough time selling their product.
That might ultimately suggest who has good fans. The fans who show up when the team is bad. And that is a difficult thing to measure.