I don't use that bike for commuting but I do have mountain bikes that I use for commuting. Three to them to be exact. The white Stumpjumper is my "main" commuter mountain bike. In winter, I outfit it with fenders as can be seen in the second picture. I even took it for a 20 mile ride yesterday and a 20 mile ride today and will use it for another 20 tomorrow...trying to fill out the year's mileage




The other two are a Moots YBB and a S-Works Epic. I have a mountain behind my work that I can ride up and over whenever I like.




All three use the tires you see on them (some components have been changed around). Full off-road knobby tires. Yes, it is slower than my other commuter bikes (I have 4 more) but speed isn't the point. The point is to use them to either break up the monotony of commuting or to enjoy a little bombing down the local trails.

I have, in the past, ridden up to 100 miles (mostly off-road) on a knobby equipped mountain bike. It takes a lot of effort but it's really not all that difficult.