Phoebe Maltz wonders about those who say they want to critique the left "from the left." If they disagree with the left about so much, why do they call themselves leftists? Why not become a rightist? Or adopt some other political identity?
Well, the obvious answer is that you're attacking the left from a position even further to the left, but from the context I don't think Phoebe is talking about that. However, I would say that she is overestimating the amount of dispute "critics from the left" have with their supposed compatriots--they probably are on the same page for most issues, and are just at odds over one (probably high profile) thing. It seems silly to strike out on one's own just because of a handful of differences.
However, at the core, I think the answer to Phoebe's question lies in the realm of a shared set of commitments or presuppositions. More than policy end-results, this is how I think most people (or at least most intellectuals) determine who they consider to be in their camp or not. People don't come up with their political beliefs in a void--they do so by drawing on rich veins of argument, writing, and schools of thought. The people engaging in the debates Phoebe wonders about are, I suspect, having an intramural affair--it is entirely possible that a right-winger might not identify with the reasoning a leftist uses to get from point A to point B, even if she too agrees with B. And on the flip side, even where they agree on end points, a person who identifies with a certain intellectual paradigm may not want to associate himself with another, even if they end up agreeing on certain end-point policies. This is especially true where the person suspects his would-be allies have made their decision based on reasoning or beliefs he finds repugnant.
For example, take a leftist who supports aggressive action to promote women's rights in the Arab World. He says that he supports this stance due to liberal notions of equality for all persons and the belief that the subjugation of women, regardless of culture, is wrong (incidentally, the belief that Western liberals and feminists wouldn't sign onto that is one of the more peculiar myths to have been perpetuated against the modern left. But whatever). Looking at right-wingers who also are loudly decrying abuses of women's rights in that region, he might still not want to join their camp, for a variety of reasons. He might think they are being disingenuous, and don't actually care about women (wait for the first Islamic feminist to start demanding abortion rights and see what happens). Even if their commitment is genuine, it might be based on stereotyping or other beliefs he might not want to be associated with ("won't somebody please save these heathen savages?"). It's not all about the end game--the journey matters just as much. Because of that, people are quite reasonable to try and convince their old traveling mates to sign on to a new destination, rather than search about for new parties who say they're going where you're going but getting there in a morally intolerable way.