As a designer, I've purchased from over 10 of the sites supported here. As a photog, I upload to all 10 of those (and a couple of others). I have my favorites but have received payouts from all of them, so I continue to upload. When it only takes the press of a computer key and pays additional $, why not...nothing ventured nothing gained.
P.S. I know I'm not the stereo typical "designer". As a contributor, I know that the same shots are not on all of the sites, so I'll take the time to look at other micros to find the "right" image...another reason to contibute to other sites (that "rejected" image from Fotolia may be EXACTLY the one I'm looking for). I also know designers who purchase a subscriptions at one site and never leave.