There is concern about McCain that he doesn't have an economic center. He (wrongly) voted against the tax cuts originally. Now that they are in place, even though he didn't support them, to allow them to expire is tantamount to a huge tax increase.
So whether its spun as simply putting his finger in the wind and flip flopping or recognizing it would be bad policy to allow the tax cuts to expire I'm glad he supports keeping them in place.
But it does again lead back to the original comment about needing to understand McCain's economic philospophy (assuming he has one).