In reply to StefanB:
I carry a pair, I find them most useful on steep uphills, also on boggy/tussocky ground to help keep balance, and on downhills to protect dodgy knees. Running is no different to walking, in that most people carry them around, waving them in the air occasionally, and get no actual benefit from them at all - if used properly, your arms should ache at the end of the day