Someone remind me why Oracle is sailing in the challenger series again?
IMHO they have no business being there.
This is total speculation, but... I'm guessing that it also has something to do with the one boat per team rule. If all teams (defender and challenger) are limited to a single boat, how do you test? Back in the monohull days, most of the teams (especially the defender) ran multi-boat programs. Allowing the defender into the challenger round robins would reduce the cost for the defender by not having to build/test/maintain a second platform.
It also makes for better media, one more boat on the water to race.