What is the purpose of adding a 300 ohm to 500 ohm resistor on the WS2812B Neopixel data line?

http://learn.adafruit.com/adafruit-neopixel-uberguide/power claims "can help prevent voltage spikes that might otherwise damage your first pixel".

In conjunction with the other warning about connecting ground first, I think it's there to prevent current flowing through the clamp or parasitic diodes between the data line and the positive power rail. It limits the current into the pin. Potentially if you have the data line connected but the positive power rail disconnected, it might attempt to power the LED through the data pin and burn it out.

It will also very slightly mitigate damage from static charge building up on one end of the data line while disconnected, and it will act as a termination resistor reducing "ringing" from signal edges. Normally termination resistors are smaller, though.