I don't understand, why people putting a "L" lens on mirrorless body It just looks so stupid.
If your mirrorless body is your second (or third, or fourth, or fifth, or sixth...) body, being able to attach any EF lens is a nice feature to have. For instance, if you travel with a DLSR and a mirrorless body, you have your compact and backup bodies provided by the EOS-M. (To establish whether that kind of setup is ideal is left as an exercise for the reader.)
For Canon, the EF mount adapter is a compromise to make the system look more flexible through the period when there are very few native lenses.
FWIW: I'm still not convinced of being to properly hand-hold a body which has no EVF (or eye-level viewfinder of any kind) with such a big lens attached.