"While it's best to be cautious of such players, don't throw out preseason stats altogether. The evidence indicates there is some predictive value to how players play even in seemingly meaningless games." - Kevin Pelton, former Sonics stat geek
The tiny sample size of preseason rather than the lower competition appears to be the major factor for why it's not more predictive. Nevertheless, it does a surprisingly nice job of showing who put up stats in the regular season.