ACtually, I agree... More than likely they have no idea that someone is waiting for a seat. If they were obviously avoiding eye contact, they're just being rude, and I have no qualms with pointing it out.

Now, since they ARE at a bar, the odds are fairly good that they WILL have drinks after. At least, around here that's the case. In this case, it may be 2-3 hours before they decide to take their conversation elsewhere. Really, it just depends on the bar.