I have a different take on the separate "oath" episodes...
In my opinion, Obama showed a lot of grace in the midst of an embarrassing confusion at his inauguration. After both men flubbed their lines, Obama merely smiled at Chief Justice Roberts and got through it without showing any sign of being perturbed. And in the face of Biden's petty dig at Roberts (in absentia, which is even more contemptible), Obama appeared to be the grownup in the room as he refused to recognize the uncalled for and out-of-place swipe at the head of the Judiciary.
In both cases, Obama served to uphold the dignity of the office. I'm no Obama fan and I vehemently oppose his policies, but I must give credit where it is due. The choice of an insufferable ass like Biden as VP is beginning to make sense now, because the contrast serves to enhance Obama's image.
Your point is very well made.
I might add that having an oaf like Biden as VP may serve to some degree as "life insurance" for Obama, since after someone as smooth and polished as Obama, a replacement that at best is jarring and at worst unimaginably uncouth, might serve to dissuade a would-be assassin.