I tried my MTB converted as a commuter (slicks, fenders, rack) for a couple of years. My commute is 12.5 miles each way, quite hilly (up and down of course) for about 1/2 of it, the other half being pretty much a flat steady speed ride, some traffic riding. I could never get my MTB comfortable for this kind of riding. It's fine on the trail where you're constantly varying your riding but for chugging along for an hour requiring relatively constant position and effort, it just killed me. I worked hard at dialing in the position, but could never get it. I don't think MTB's are intended for this sort of stuff, although I know that some make it work very well. The road-style cross bike commuter I have now is 1,000 X more comfortable and enjoyable.

So I built up a cross frame into a commuter with 32mm tires (and fenders and rack). Just for reference, I also have a very light weight carbon fiber, racing-tired road bike which I also love.

This type of "road position" commuter is so much more better for me compared to the MTB. Again, for long-ish steady state riding, I just don't think MTB's are intended for that and I just wasn't able to make it comfortable although it works great for trail riding.

But on the question of the tires: I LOVE those 32 mm tires! What do they do that the 23's on the road bike don't (by the way the high end 23's on the road bike are surprisingly comfortable and durable, no complaints there).

The wider tires:

give you more carefree riding. No need to avoid gravel patches, you can divert from the pavement onto grass, dirt or gravel without feeling unsteady, they are much more steady and comfortable in potholes, pavement cracks, transitions, etc.... and because of that, a lot of the supposed loss of pure speed really disappears. You just don't slow down or worry about your path/route. I have never had a flat (about 1000 miles so far) and have never given one thought to road debris, including glass. I don't seek it out and make normal efforts to avoid it, but don't worry about it and haven't had a problem.

Plus, yes maybe the bike is slower.... in some situations, but not all! And some of that has to do with the overall weight of the bike, not the tires.

It's about 5-7 pounds heavier (mostly from the large tires, fenders, rack, trunk bag; the bike and wheels are only a couple pounds heavier), so it is a little slower going up hill.

I don't think it's much slower at all on the flats on smooth pavement. OK,call it a TINY bit slower on the flats.

But I believe it's faster - AND more care free - on rough pavement, transitions, avoiding obstacles, etc. It's hugely faster on any sort gravel, even smooth gravel. But rough loose gravel? No comparison, the 32's can handle it, the 23's are terrifying and/or I just can't ride on it.

Another thing that contributes to overall speed and mitigates any supposed increase in rolling resistence is the fact that I run the 32's at about 60 lbs (compared to 90-100 on my 23's). This means I don't notice every little crack and bit of gravel on the pavement, and am much more likely to roll on through rather than avoid this stuff or slow down for it.

So for commuting, I think the wider tires actually make my commute faster and easier.