I would never go to a restaurant where reservations or needed, or that had a wait at the door and sit at a table just to order drinks. I think it is rude to those waiting to actually sit down and eat. When they are not busy and have lots of open tables, or if you sit at the bar then I think it is fine. I don't think the rules of common courtesy should be any different at Disney. I would never walk into a restaurant that requires an ADR and take up a table just for drinks. I sugest that if you really want to see sci-fi, go for shakes and burgers, or get shakes and a couple of apps to share.