MicrostockGroup

Aerial View is in the CV, and that's pretty close. 

Not at all - these have extremely little, if any, relationship to each other.
Aerial view is a view from above, e.g. from an aeroplane.
Aerial perspective is the effect of atmosphere on light, whereby more distant objects seem paler and more hazy than objects in the foreground.

Back to the original topic -
I was in a doctors office with my son today and looking at the magazines.
I picked up a copy of Parenting (www.parenting.com)
In the March 2010 issue they use images from: Shutterstock, Getty Images, Corbis, Veer, Blend & Alamy.
So in this case I guess the answer is no, they shop around for the image that fits their need.

In the magazine yes not so much in the online version - there it looks like they mostly credit the agency, but I din not delve into the online magazine too much.

I used to purchase from dreamstime when I worked as a graphic designer.

I wasn't a contributor then, and the company I worked for was not aware of dreamstime. I introduced dreamstime to them and although I no longer work for them, even years later, I know that they still use dreamstime.

I think it is more of a habit than loyalty. They are just used to buy from dreamstime now. I still see some friends from there and did introduced them all the websites I contribute to, but they didn't change their habit. Maybe because they already have a dreamstime account and don't bother to create another one, with another site.

Only if they don't find what they are looking for at dreamstime, they may use other sites which is a rare happening.