For the same reason people use environment cube reflection mapping: If you can't have the real thing, go for the second best. Also, I would imagine that this method wouldn't require too much coding.
I've just now came up with an even easier way to do it, and all I would need is the ability to use the alpha channel of the environment cube map. Sounds like a small alteration of the environment cube reflection shader, but as said before, I don't know much about shader programming.