Well, that adds into their cost as well. It's not necessary, but this option is good to have. It does reduce the rumble when you turn up the bass, and it certainly separates the speaker from the water completely. The factory RAV4 speaker adaptor shield the factory speaker from the water dripping, but with the aftermarket speakers, sometimes these have deeper installation depth. I guess the factory adpator won't be good enough.
I feel safe to leave my speaker in a foam cup =D