ECV would allow a region owner or manager to define a space then fully control all of it's environmental aspects, such as time of day, sun position, sea level, gravity, etc. Sort of a region within a region. Except for gravity, all these settings can be changed on a region-wide basis. But ECV would enable a portion of the space within a region to act like a separate region. Thus a "skybox" could be created with genuine Linden water and a fixed sun to simulate land at twilight, for example, without effecting the environment in the rest of the main region (Web).
I thought about this concept again when I had to create a massive reflection pond with a 30 meter water fall to "sea level" on the Isle of Lesbos. I didn't mind using four megaprims to make the pond, but even with clever textures the water isn't quite as attractive as "natural" Linden water. And four more scripts were added to the sim load to animate the water. I like the solution used in Lionheart but it would have been impractical at Lesbos.