The argument for racing has always been that the constructors learn things about the cars through development that would use the same resources through other means.  In the meantime, the money generated by the events helps to offset the costs of this learning process.

Now I realize I'm being the devil's advocate to even suggest these things, but in a debate, these are the arguments you might hear. Your environmental points make sense, but most sports generate so much unneccessary waste that to pick on NASCAR only is rather selective.  They seem like an obvious target because of the fuel consumption, but the comparisons might be closer than you'd expect if you had reliable figures at your disposal.