Here's something I've read a few times, including in my comments:
I'm still trying to get my head around the idea of specifying arbitrary BBOXes for image requests in REST format: seems utterly uncacheable, and thus against one of the REST goals.
If you seek to build a high performance, highly scalable map image or coverage service (such as the one Google needs to support Earth and Maps), exposing an infinite number of view resources is a design anti-pattern that you must avoid. (Note: a WMS doesn't expose a truly infinite number of resources, but a number N approximately equal to 10 to the power of 4W, the precision width in digits of the bounding box coordinates. N approaches infinity fairly quickly: MapServer exposes something like 1066 unique views of a world map.)
If you want to cache effectively, you must switch your design over to a finite number of view resources. Tiles, in another word. There are other advantages to the REST architecture besides cacheability, so don't get the notion that REST fails for large values of resource number N.