Marc cedes too much to the Frumians, if you ask me. It isn’t just that Scott Brown is more conservative than Linc Chafee, or Arlen Specter, or whomever. It’s also that, in the brightest issue on the field, healthcare, he appears to be a reliable Republican. By contrast, the RINOs give the impression that they’ll vote with the Republicans, but only when it’s convenient for them.
More importantly, though, the accusation of a right-wing search for “purity” mixes up distinct segments of the political landscape. If a Republican can beat a Democrat and offers a more-right alternative, conservatives will back him or her. They may try to defeat such candidates in primaries (with adjustments made for the extremity of their moderateness and the likelihood of victory in the election), and they’ll battle with them when defining baseline opinions of the party (as in the platform), but purity has never been a refusal to work with with others as much as possible. It’s been a refusal to redefine the core of the party to be more amenable to them.
Moderates like Frum want to sell moderateness qua moderateness, not establish a neutral right-leaning playing field on which all Republicans can interact. Indeed, he illustrates perfectly the attitude that conservatives despise in moderates: His goal clearly isn’t to secure a place for his relatively liberal fellows, but to overtake the party in order to advance his own ideals. Moreover, he’s apparently delusional:

It would be a travesty if Brown’s victory is seized upon as a victory for anger, paranoia, and ideological extremism.

I don’t think it uncharitable to interpret “anger, paranoia, and ideological extremism” to be Frum’s characterization of Tea Party types and farther right conservatives. If that’s the case, he misses the obvious fact that Brown is, indeed, benefiting from a national surge from that group. He is benefiting from the angst of conservatives. Without that angst, and without Brown’s agreement on central issues, such as the healthcare bill and cap’n’trade, there is no phenomenon in Massachusetts.
It is likely helping Brown, locally, that Massachusetts moderates find they don’t disagree with him on every issue when the national attention pushes them past their suspicion of Republicans. But Brown isn’t winning because he’s running a campaign as a moderate. He isn’t having million-dollar fundraising days because he’s tirelessly shaking hands and interacting with voters. He’s winning because he’s part of a national backlash.
Moreover, as Boris Shor puts it, in a post with a very interesting chart showing the political leanings of parties in all fifty states:

What this shows, however, is that the conservative base in the United States, far from dragging their party moblike into an unelectable extreme, has made the decentralized decision to support the realistically best candidate they can relative to the context in which he’s being elected. The 23rd special district election can also be seen in this light; throwing Scozzafava overboard made far more sense in the context of that electorate.

That, to my ear, is the argument being made by Rhode Island Republicans who wish to close the primary. They want to pick the best Republican candidate they can without having unaffiliated voters ensure a choice between two liberal “moderates.”
In the context of this debate about “Northeastern Republicans,” Rhode Island’s row on Shor’s chart has interesting implications. According to the data, between 1995 and 2006, Rhode Island’s elected Republicans were the most liberal in the United States, while the Democrats were pretty much dead center in the national range for their party. In other words, whatever strategy one might derive from Scott Brown’s success does not necessarily apply to current circumstances in our own state next door, where Republican “moderates” are far left and elected legislators overall are left of center. The fact that the RIGOP has had so little success in the General Assembly suggests that more unnatural liberalism is not the answer.