I've been riding since the mid 60's, and even back then many of us riders would acknowledge other riders with a wave of some kind. I'm never riding so hard that I'm afraid to take my left hand off the handlebars to give the two fingers pointing down to an oncoming rider, or group of riders. And if I'm in a situation where both hands are needed on the handlebars I'll nod my head or otherwise acknowledge the other rider. It costs me nothing and hopefully being recognized by a fellow rider will make him or her feel good. I see it as a sign of respect to other riders, just like always stopping to see if assistance is required if another rider is stopped on the side of the road. 99 times out of 100 the other rider is just stopped for a break or some other reason that does not requires assistance, but I have stopped for that one rider who required assistance and I was able to provided it, so all the other times were worth the little effort. We're a small minority brotherhood (which includes females) and recognizing other riders or offering assistance is a great way to support our small community.