The cost is so low that I am sure advertisers are drawn to the concept just to see if it works. How low? Try $1 a pixel for five years. In the case of PixelBay the company touts the fact that it will drive traffic to its website in large numbers. A look at the Alexa Rankings of the site show that while traffic spiked for a while, it is on the way back down now.
Here is an article on the topic worth reading and this is where I found this quote that I agree with:
"Things that have no benefit to the consumer don't last very long. They can survive on curiosity and novelty for only so long," said Jim Nail, analyst at Forrester Research,
In short, unless the company and others can find a way to attract people to the concept will most definitely remain as a fad.