On some level, this makes sense. If Romney is the ultimate nominee, the KosKidz wil have forced him to spend more money (and potentially alienate Santorum supporters he'll need in the fall) in order to win. If Santorum is the nominee, Kos foresees having a field day.
The reason for the glee over the potential for Santorum as GOP nominee? It comes down to what Matt Lewis (once of Townhall and now of the Daily Caller) characterizes as "contraception conservatism." Santorum's willingness to talk about delicate social issues is admirable (as the author of a book called "Prude," believe me, I know it's not always easy).
But the effect of Santorum's willingness to opine vocally on issues such as the use of contraception is to open a chasm between two vital parts of the Republican coalition -- those whose emphasis is on social conservatism, and those whose emphasis is on economic freedom. The latter tend to be somewhat more libertarian in their leanings and uncomfortable with what they perceive as government intrusion into private decision making, whether it's from the left or the right.
Dividing social and economic conservatives is a problem, because without both (along with "strong defense conservatives") a Republican cannot win (unless the incumbent is fatally flawed AND the nominee demonstrates an almost uncanny ability to win independents -- a dubious strategy, since most of the money and grassroots enthusiasm comes from the base . . . or doesn't, as John McCain learned). (Some might argue that Romney's various past apostasies do, in fact, alienate social conservatives -- but that argument is hard to square with Romney's CPAC victory and various polls showing that he performs fine among garden-variety conservatives.)
The problem would be happening in reverse if Mitt Romney were still pro-choice. In that case, he would be dividing the party by alienating the social conservatives, just as some of Santorum's views are alienating the more libertarian wing of the party. As Byron York notes, part of Santorum's challenge is to prove that he's a better candidate than he was when he came across to Pennsylvania voters as "arrogant and headstrong, preachy and judgmental." His willingness to opine (and history of opining) on divisive social issues like contraception (where even hard-core conservatives can disagree) gives pause, as politics is nothing if not a game of addition.
Ultimately, if we are serious about defeating the President, the question becomes: Although both Santorum and Romney have flaws, which of them is likelier to be able to hold together the Republican base AND attract (or at least not frighten away) swing voters who will be deluged with misinformation by the Obama campaign and its allies in the MSM?