Bullets arc too, the only difference is that a paintball drops a lot sooner than a bullet. The red dot will work for anything close to midrange, anything further and the paintball will end up anywhere. For me it has some use because it assists in showing me where I'm aiming my barrel. Sometimes I get disorientated with my gun so the red dot is handy to get me back on track. It's also good in a close quarters skirmish where you need to aim for a really small target like a toe poking out from cover. I have used my sight numerous times to score hits on other players.
So it's not entirely useless.