Do you really think the market and customers need one more proprietary/specific mirorless format?
The only interesting thing if Canon join Mirrorless would be to see they join micro 4/3 group, even i doubt this could happen.
This would stregthen considerably this format (so this would vallue the Panasonic and Olympus offer, but also this new Canon offer, compared with all other competitors), and Canon users of a Canon micro4/3 body could benefit for the wide range of quality lenses yet available on the market. Canon would not have to develop a complete range of lense for the launch of their product but can do that little by little.