When my local Borders bookstore had its going out of business sale, I snatched up a bunch of Photoshop magazines. Some of these magazines are imported and typically cost $15 or more but at 50% off they were too good to pass up.
Inside a few of the magazines were a few ads for microstock agencies. What I found disappointing was that the main message of the ads seemed to focus on low, low prices. Seems like most microstock agencies or at least their advertising department seem to think that price is the most important thing to microstock customers.
I have a marketing background so I know where this battle to the bottom would ultimately end up. Its like pushing the dollar menu at fast food restaurants.
You don't see a company like Apple advertising their product as the lowest price. Rather they push the benefits of their products and how their products are better than other companies. Anyone can jump on the lowest price bandwagon but quality, selection and exclusivity is harder for a company to provide and ultimately more important to the customer.
Dreamstime has an opportunity to rise above the bottom feeders offering cheapness by its exclusive contributors. Being exclusive with Dreamstime is like a partnership while offering your work to everyone just makes it a commodity.
I don't think the ads talking about low prices addresses the microstock customer at all. The most important thing to designers I imagine is to quickly find the image they need. They are not looking for to save a few cents on an image they just want the right image. They also want to find a unique image, not one used by everyone and their brother.