Wow! I never know these existed. Assuming they had favorable optical quality, that would be fantastic.
I can see why Canon might be reluctant to make one of these. It certainly wouldn't eliminate all of the advantages of Full Frame, but it would make it a lot closer for those shooting APS-C.
Here's hoping that Metabones, Sigma, Tamron, Tokina or someone comes out with one of these for EF to EF-S, in the not too distant future.