I think it's a fad or craze that has over-stayed its welcome.
When I got into bikes in the 1960s there were few of them touring on the open road. Heck, other than BMWs and a few Honda Fours not many could tour, at all. The odds of meeting one on the highway logging major miles was quite rare.
In fact, as late as the early 1990s my wife and I routinely saw broken-down Harleys along I-94 going to South Dakota.
However, during the past five years I've only had to stop for one bike--and he was just out of gas. Bikes have become more reliable. Yes, I've made upgrades and I will continue to do so. I snapped a throttle cable a few years back. (Harley already has a "fly by wire" feature on some of its line.)
But hippies are now corporate execs and drive their spoiled children to soccer practice in the family Prius. They don't flash the "peace sign" and remark "Later, dude."
I don't think we should 'wave' either. But yeah, I do "two low" to fellow Harley riders. If anything it shows respect to guys who can save money or hold a real job.