It's a table service restaurant, you can't just go in for drinks, you should really be eating too.
Seems a bit rude to people that want a full meal to book up an entire ADR and table just for a shake. Maybe not during off times, but it would be a bit much during peak dining times, IMHO. That's just me. I wouldn't do it. Not saying others can't or shouldn't. Just something to think about. I know I'd be annoyed if I couldn't get an ADR somewhere only because someone booked them up just to order a milkshake.
I've done shakes and fries! The servers are not thrilled, but I asked a CM when I made ressies and they said, "You don't have to tell us what you're eating...you can order whatever you like!"
We went after Fantasmic one night....very fun!