I'm going to go with Matt's answer of "what do you do with him if he lives?"
If you think about it - when Vader asks Luke to remove his mask, sure Vader's breathing was labored, but it still worked.
You mean to tell me the medical frigate wouldn't have had a facility where they could have cared for him? Of course it did.
So instead of taking off Vader's mask, Luke could have dragged him into the shuttle and plotted a course for the medical frigate, got on board and brought him before the medical droid.
But then what?
Mon Mothma: Well, thank you all for coming to this after-the-Emperor's-death-so-now-what-do-we-do meeting of the Rebel alliance. As many of you may already know, we have a very special guest this evening... the father of Luke & Leia... the guy who tortured several of our comrades to death... the Emperor's former right hand man... Darth Vader is with us, let's give him all a big hand for throwing the Emperor into the reactor!
Um, yeah, doesn't really have as much gravitas as him dying to save his son.