This last weekend, I went camping with all of my wife's cousins.
I would have taken my road bike to explore the back roads of the Sierras, but our campsite was down a very bumpy five mile dirt road. So, as much as I wanted to get a good workout at high elevation with my road bike, I figured my mtb would be a lot more practical for this trip.
So I took the bike out on the second day of the trip for a few miles on fire roads and maybe some trails.
Holy crap. Did you know that hills on trails are steeper than hills on roads? There were places where my granny gear just wasn't granny enough. And when I could keep my wheels turning, I was going so slow that the bike was impossible to control. And when I did get enough torque to get going, my back wheel often spun in place. Trust me, I've never made my rear wheel spin on my Trek 1000, even with my 12-26 Shimano Sora drivetrain.
Then, I had to go downhill. I have more grey hairs now. There were rocks and stuff in the road. If I went too fast, I feared that my bike (and I along with it) would be bounced around like the time I tried playing with my Hot Wheels on a cobblestone walkway. If I tried to go slow, it was like I was going to go over the handlebars, provided that the wheels of my bike didn't slip. Which was another problem. Did you know that dusty dirt roads are actually slippery? Many times I tried to stop, but kept sliding.
I did find a good use for compact geometry on mountain bikes, however. It makes carrying your bike over the "technical" sections of single track much easier.