Let me get straight to the point. I think white crayons are useless! I am not a crayon expert, but the only time I have ever seen them used was back in grade school when kids did Halloween art projects and drew ghosts on the black construction paper. Aside from that one scenario, I think they are just a waste.
I suppose if someone was going to kill me and I wanted to leave a secret message that the murderer wouldn't notice, I could always use a white crayon to write who my murderer was. Then when the CSI guys came to the crime scene with their ultraviolet lights, they might be able to read the message, but I'd rather not focus on such a far fetched scenario to try and find a good use for white crayons.
Putting a white crayon in the crayon box is kind of like buying a dozen donuts and then realizing that one of them is actually an onion flavored bagel. In case that great analogy didn't work, imagine buying a bag of otter pops and then you notice one has no juice in it, but is just filled with air. I think I've made my point with these two powerful and persuasive images.
I'm going to start a new trend. When you are looking for a descriptive phrase like as "clean as a whistle" or "as cute as a june bug", (both of which are horrible I might add), I suggest you start using the phrase "as worthless as a white crayon". I'm pretty sure it will catch on.