That system is only for Presidential Primaries. There’s a primary for most positions all the way down to dog catcher. Getting a partisan seat gives you a voice in the state party and possibly the national if your seat is high enough. Get enough Congressional seats and you control the special delegate swing in Presidential Primaries. Might they change that system as a progressive movement gets more seats? Sure, and they can have another riot outside their national convention too.

The very thing you call rigging is vulnerable to a tea party style primary challenge.