The argument in favor is equally simple: if Georgia had been part of NATO, Russia never would have invaded. The commitment itself would have prevented war in the first place.
Unfortunately, there's a wild card here: the Russians have obviously been itching for war with Georgia for a while, but in the end, it was Georgia that sent troops into South Ossetia first. Did they do this because they felt they had a tacit commitment for help from the United States? Would NATO membership have made Mikheil Saakashvili even more impetuous than he already is?
Hard to say. But one of the reasons we have no formal defense treaty with Taiwan, instead maintaining "strategic ambiguity," is that we believe it restrains Taiwan's options. If they were guaranteed American help, they might declare formal independence from China and touch off a war that no one wants. In this case, longstanding U.S. policy holds that the lack of a treaty helps keep the peace.
Obviously that didn't do the job in Georgia, but by all accounts Saakashvili felt that recent events suggested he could count on Western help if he took on Russia. If he hadn't been led to believe that, maybe he would have held off on baiting a neighbor he knew could crush him easily if it wanted to. Perhaps a bit more strategic ambiguity might have been in everyone's best interests here.