Like GD, I haven't had problems with the plastic spurs either. It's all about a consistent and proper mesh. They are quieter, cheaper, and lighter as well.
Also, back in the day, your typical pinion would be eaten up in short order with a metal spur and there was a shortage of reliable hardened steel pinions, so most everyone used the Kyosho spur with regular pinions.
During the "Kyosho spur" days, I started a discussion on this topic. Since then, things have changed:
http://rc-monster.com/forum/showthread.php?t=4383