Several tips: Something that can take a rack. Something with a lockable fork. Avoid dual suspension. Avoid anything too heavy. Unfortunately, these are often cross purposes. Finding a mountain bike that can take a rack for carrying your stuff is difficult. There are a few but they can be hard to find. Jamis XC Sport bikes have rack mounts and I think some of the Treks do. You just have to look. Disc brakes make mounting racks more challenging but not impossible. You just have to be creative.

Lockable forks are fairly common but, if you really want to ride off-road, you also should be looking at higher quality forks. However, those don't usually come on bikes that have rack mounts...that whole cross purposes thing

Weight is also a problem since most of the bikes that can take racks are on the lower end of the bike line and, thus, end up with weightier parts.

Dual suspension can be handy in some situations...works wonders in ice and snow pack.. but it's usually too active for most commuting and you really can't find a dually with rack mounts.

There is nothing wrong with using a mountain bike for commuting. I do it all the time. Yes, it's slower but if you can find an off-road option, it's much more fun than "regular" commuting.

If you haven't tried a current short-link full suspension bike like a Santa Cruz, I really recommend it. They have the kinematics figured out now and they don't pogo at all as you pedal, or jack up with braking. The torques and tensions of pedaling and braking counter the movement.

The problem with using one as a commuter bike is that they aren't set up to carry stuff. I have a 2003 Specialized Epic (and a 2005) that doesn't pogo or jack up when braking and it does make for a great commuter bike. It's just hard to carry the stuff I need for commuting.

They are both great mountain bikes...perhaps the best dual suspension bikes I've ever ridden... and the 2005 I chose specifically as my all around bike for leaving at my daughter's house but they are limited as a vehicle for bicycle utility.

A good used hardtail still makes a better commuter bike like this one

DSCN0377 by Stuart Black, on Flickr

Try a DW-link or VPP bike, I think you'll like it even better. The VPP patent expired and now they're proliferating. Bikewolf asked for a mostly-MTB and occasional commuting bike to supplement his hybrid, that's why I'm arguing for more fun. You can always take a backpack on your MTB day. I'm actually using a backpack full time now even though I'm not a fan, because on daycare days my rack is occupied by a kid seat and on non-daycare days I'm taking the trail on my new full squish bike. It's not prohibitive to skip a rack.

Depends how far you're commuting, there's a reason road bikes differ from mt bikes. OF COURSE you can commute on a mt bike, but might not want to.

I inherited my son's Kona Blast, Marzocchi front fork. I equipped it with front and rear racks. Yes I could ride it all day on pavement and yes I could carry 50+lbs between four Ortlieb panniers on that bike but.......

The stiff aluminum hardtail frame but that back wheel close to the seat - a harsh ride in back. Those short chainstay made heel clearance on the rear panniers iffy.

The front fork has no lock out, yes you do lose energy pogoing the front shocks during normal riding on the street. The short wheelbase was relatively unstable and became worse when the bike was carrying a significant load.

The '95 rigid steel frame Specialized HardRock I'm on now suffers from none of the above deficiencies.

Mike

I think you have that backwards. Bikewolf's original post:

If he were wanting a mountain bike that could be used occasionally for commuting duty, a double could make some sense. The issue, however, is also cost and risk. A hardtail mountain bike with a good fork which also has the ability to take racks is fairly inexpensive. Even if you upgraded the fork to a much better fork then you find on the lower end bikes, it's still relatively inexpensive.

Any dually worth riding off-road is going to cost much more than what a hardtail does. The Jamis Trail X cost from $450 to $750. A Jamis Dakar cost $1400. The Dakar is also going to be much more of thief magnet than the Trail X.

I use a Revelate seat bag when I use my Epic as a commuter bike. But that's only during the summer. The issue that I usually run across, especially in the shoulder seasons like now, is the lack of ability to carry excess clothing I need going in to work but don't need going home. For example, I had to wear a long sleeve jersey, tights, jacket, gloves, long socks and ear warmer on the way in today, in addition to my shorts and short sleeve jersey. It was about 35°F.

On the way home, I'll be wearing the short sleeve jersey, short socks and shorts because it's going to be 80°F on the way home. A backpack or even my seat bag just aren't able to handle the excess in addition to my work clothes. The bike in my picture above can take a trunk bag that has drop down sides when handles the excess nicely.

I'll grant you that the dually is outstanding in snow but it's just not as practical as a hardtail for carrying stuff. I actually have a Moots YBB that serves as my studded tire bike but I've fashioned a rack system for it that makes it as practical as the Dean.

Nay. I'm sticking to the Epic. The inertial valve on the rear shock locks it out better than any other system I've tried for riding on pavement and it's extremely active when it gets hit from below. I have yet to find any conditions that would make me want to change. It works as advertised and it works in the background. I don't have to flip levers or even think about it engaging.