Josh Marshall asks who does McCain need around him to make him feel comfortable enough to go.  Two thoughts.  McCain's affection runs deep for Lieberman; I think he thinks of him, even treats him like a 'wing man.'  He's certainly served McCain in that way.

You not only need people around you that give you a comfort zone, but you also need people not there that make you anxious.  I think that Palin actually makes McCain nervous.  He know who she really is, how dangerous she can be (hence keeping her under wraps and away from the debates).  This is especially the case on foreign policy.  As an attendee of the debates, rather than the main act, it is especially unpredictable and precarious, as run ins with reporters is a distinct possibility for Palin.  So, I think if he goes to the debate, he'll have Sarah off doing something appropriately insignificant.