^I thought about it, but that's not the point. The point is that still is a rare weapon that still is strongly associated with Jedi, and it doesn't seem like a good idea to use it when Imperial troopers are around. Just imagine that troopers came into the bar just 5-10 minutes earlier and saw a lightsaber. They'd probably stop them to at least question, or try to arrest right away. Obi-Wan could probably brainwash them again, but even this power has limits, since when he saw that troopers were coming in the bar, he decided to leave right away. Now imagine if troopers reported to Vader (who as we know used to hunt down remaining Jedi). Just imagine if instead of only troopers we also got Vader coming to docking bay 94 if he made it in time. My point is that it's a HUGE risk just to deal with some a-hole in a bar, not too clever for a hiding Jedi.

The real reason for it I think is... it's a movie, and it's more entertaining to see.

One more thing that is head scratching if we take OT on it's own. Luke lives with his aunt and uncle, right? But we don't know how they are related. From the way they talk about Anakin (Beru calling him as simply "Lukes father", not "my brother" or anything) we can assume that Owen is Vader's brother (or at very least in-laws). So they HIDE Luke from Vader... at his brother's home. We can only assume that Vader can't give less turds about brother, but nothing more, for a reason why Vader doesn't visit him or anything. With prequels however we see that 1)they are step-brothers and hardly know each other personally 2)Anakin had only bad memories related to Owen and Beru (Shmi's death). So within 6 episodes, we clearly see the reason why Vader doesn't visit brother on Christmas :)