Very interesting topic. I always thought the spoiler was to change the ariflow on the rear of the vehicle. The change in ariflow was to, essentially, help hold the rear end down and gain a tiny bit of traction in wet weather.
I've used my rear wiper matber two or three times in the past year or so, even with the spoiler, Those were generally, as pointed out, to clear rain that blew on while the RAV was parked.
I liked the look of the spoiler, I was not particularly thinking of any other benefits. My second vehicle back, a Chrysler Laser, had a spoiler, and I hated it. After the first year or so, the stupid thing rattled terribly, never could stop the rattle. I was a bit concerned about the RAV doing the same, but, knock on wood, so far, so good.
Best
Ken