I've written about this on my blog in the past and the core piece of evidence in the 'against' case was one of the top-selling microstock contributors once deleted 600 of his poor performing images from iStock and noticed an instant and significant drop in revenue.
Of course that only demonstrates it's a bad idea at iStock, so it could still be helpful at other agencies.
Also, since I started monitoring my sell-through rate I've been noticing many sales in the recent sales pages at iStock, Fotolia and Dreamstime where the download count is 1 - indicating it's the first sale of that image. Some are new and many are old. It's the long tail at work. The image might only sell once in it's lifetime, but if your portfolio is large enough this can add up.