We looooove our hiking poles for hikes with climbs and most especially for descending. We've done some hilly hikes without them and always regretted it! They help going uphill to be sure, but even more for going downhill, protecting the knees, and as Crankin aptly notes, for stability. However, they can also cause a fall if the tip gets wedged in between rocks...don't ask me how I know this! 
Still, if you are careful with placement, they help way more than they hurt. I've hobbled down trails with a sore knee before that there's no way I could have done without 'em.
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow