I switched to my mtb last year for commuting, primarily because the roads that I use on the ride in are crap for bikes! Did not want to have to deal with a flat that early in the morning....
Also, the mtb has shocks - very important and appreciated.
Yes, it is slower but I put slicks on so that improved things. Simplifies things also - the panniers clip on easily to the rear rack so it is ready to go in a flash. That way I don't have to transfer stuff between bikes or use a backpack like I did when riding the road bike in.
Serendipity
"So far, this is the oldest I've ever been....."