Yeah, I think you may actually be missing the point of the "wave". It is kind of like an acknowledgement of a family of sorts. I realize wrx's are mass produced, but it is still a niche car. Not everyone drives one, or likes them for that matter. It's not an exclusive club, but it's a shared interest. Now probably most of the people that do wave and or return a wave are going to be your enthusiasts, even if it's not modded. The other wrx owners are just in it for maybe a little more fun or whatever reason. I think once someone who is "uneducated" in the wave gets a wave, they probably think "hey that was cool" and start waving themselves and returning the gesture. I also ride motorcycles and bicycle, so I'm very familiar with the communal waves of different ilks. When I bought my wrx I got my first wave on the way home. I actually didn't realize the wrx community did that. I knew about the jeeps too, but never owned a cj or wrangler. But I knew what to do and just rolled with it and thought "that was cool!"
*Sorry Amber I was backing you up, but the quote didn't get the whole thing!