Also, Obi-Wan doesn't say that he won't help Luke, but that he can't. In other words, Obi-Wan is now only a ghost and not able to directly affect the living world. Obi-Wan really stresses that Luke will be alone in his fight against Vader—alone and untrained. Yoda looks shocked when Luke thinks a moment and says, "I understand."
A minor point, but an interesting one. :)