I do the "The wave" to anyone on a motorcycle, including scooters, unless It's dangerous to do so, heavy traffic, mid-corner, etc.
+1 I also typically wave to farmers, old folks on porches, cops on bikes or horseback or on foot, people out in their yards when I'm riding through their neighborhood, and most importantly KIDS
. I sometimes toot the horn at kids, too. If a kid walks up to me when I'm parked with my bike, I always find a little time to say hello and wave to the parents to put them at ease. Never hurts to give people a reason to associate motorcycles with friendly people. Amish farmers are reliable wavers, so are guys riding on really cold days.
As far as the rationale behind the wave, I find people in some other vehicle types wave to each other: Miatas, Jeeps (CJ/Wrangler especially), antique cars in general (and people driving the same make or model in particular), etc. 30 years ago I had a buddy who drove an MG who waved at everyone else in an MG...they all waved back.
Here's an unscientific observation based on nearly three decades of riding motorcycles (and waving to my fellow riders): people wearing helmets, whether in states that require them or not, are much more likely to wave back than those without helmets. This is reliably the case with Harley riders. In Massachusetts, for example, (helmet law) I find Harley riders wave more often than not. Cross down into Connecticut or up into New Hampshire, however, (no helmet laws) and Harley wave rates plummet. Maybe they're feeling too badass to be friendly? Among Harley riders who do wave, I especially like the ones who "wave" by pointing one finger towards the ground. ("Yup, there's the road, I see it, too.")