Given the simplicity of putting on nail polish, the drawback of wearing it would be pretty small. Remember, while the rate of drugging rapes is small across the whole population, there are a lot of women going to situations that they know have an inordinately high likelihood of someone raping them, and Not to mention, they are going already going into it with the intent of drugging themselves with a drug that induces poor judgement.
As with any security measure, it is a question of balancing convenience, cost, and risk. Going to a 3 year old's birthday party? Don't need it. Going to a frat party where you already know a couple of the guys are rapists? Yeah, the nail polish is probably a good idea.