It's not so much that having a nice car helps you, but rather having a not-so-nice car can hurt you, especially in the luxury market. Buyers have commented about other agents "gee, he drove up on a Honda Accord, I just don't see how he's going to share our tastes and appreciate the finer things."
I happen to already like nice cars so I drive them anyways, in fact just got a new one a couple weeks ago.

But since I work the high end market, if I didn't already have a nice car I'd definitely have one on the shopping list. And by "nice" I don't mean a new Honda or Ford, I mean something that could be considered a luxury car and I think we all know what that means.
By the way luxury cars don't have to be expensive. You can get a great used Lexus, Mercedes, Jaguar, etc for under twenty grand including a comprehensive extended warranty.
Is this fair? Is it how things should be? Would Budda approve? I don't know, but I can guarantee you that if you're selling penthouses and your client sees you pull up in a Toyota Avalon you've just shot yourself in the foot.
Here's a list of some nice cars from the $20Ks and lower:
http://tinyurl.com/drw3d