It is NEVER OKAY to take an illegal taxi (white plated vehicle). You are not covered by the driver's insurance. White plate vehicles are not insured to carry paying passengers and should a serious accident occur, since you were participating in a crime (hiring an illegal taxi), you can be held liable for damages as is the driver. Suggesting otherwise is terrible advice and should never be done... period.
Absolutely great advice. You don't want to get involved with the Jamaican legal system if you're a visitor.