Why have a ranking system in Aikido? So...what's the point?
Positional power (authority) comes to mind. So does money.
Off the top of my head: Sambo/Sombo, San Da/San Shou, boxing, kickboxing, muay thai, wrestling (freestyle, folkstyle, greco, catch-as-catch-can) don't have rank at all. For the most part: bjj, aikido (as you noted), jkd and judo don't have rank specific technique.