Obi Wan may also be looking at the fact that if Vader is using "Pain and Torture" to get Luke to come find him, Vader may very well kill Leia to lure him in (not knowing who she is).
So you have one-half of your chances at saving the universe, running off to save the other half. If Vader has Leia AND he defeats/turns Luke, then he was the last hope.
Does that make any sense?