I think the brutal and long primary served Obama well in 2008. But, history tells us that incumbent Presidents standing for reelection who have serious primary challengers, don't fair well in the general election.
If Obama were to be seriously challenged, it would be from a candidate that appeals to the far left, not the moderate middle. Obama would have to tack even further to the left during the primary, increasing his exposure to attacks from the middle and right in the general. That's not good for a man who, if he has any chances of reelection, has to win back the moderates he's alienated. A primary challenge hurts him.
Having said all that, if he's still enjoying 95%+ approval ratings with blacks, he can't be dethroned - not by Hilary, not by anyone. Blacks are just too important a demographic in Democrat politics.
Good points.