I never thought much of trekking poles - until I was a few days into my "trek of a lifetime." The plan was to walk from the end of the road in Jiri up to Kala Patar (Everest basecamp) and then back to Lukla for a flight out to Katmandu.

I can tell you that after my second 1,000 meter decent (right after a 1,000 meter ascent) my knees were crying out for help, and the poles made a HUGE difference.

Currently, if I'm going out for a dayhike, even over moderately rough terrain, I'll leave them in the car. However, if I'm carrying a heavy backpacking pack I find them very useful for maintaining balance, even on easy terain. Even though my arms are more tired than they would have been without, I am usually less fatigued as my core muscles have had to expend less effort keeping me fromm falling over - once you start to lose your balance, it takes a lot of effort to stay up with a heavy pack.

One time I try and use them no matter what is if I'm carrying my son in his backpack carrier. If we're on a trail and he's on my back, I'm using the poles. I just have no interest in finding out what kind of injury he'll get if I fall while carrying him, and I'm less likely to fall if I'm using the poles. Of course, if we're just walking around the mall or on a paved path at a park, I agree that poles would be rediculous.

Finally, I have found that using two poles reduces strain much more than using just one. When I've gone out with just one pole, the stresses on my body (and specifically my knees) become unequal, and lead to additional aches and pains at the end of the day.

Hope this helps. If I was going out on a multi-day backpack trek over rough terrain, I'd absolutely bring poles along.