Well, let's think about this way.
A lot - and I mean a lot - of people agree with the most common critiques of capitalism made by socialists. Our analysis is often right, they say, but our solutions to it are wrong. And the majority of these people do not side with us, because they need both the diagnosis and the proper medicine. Without both, there's no commitment from them, and there may even be hostility if our remedy is worse than what they consider the disease to be.
Our attitude towards the SJWs, except for a few minor fringe groups who are essentially anti-capitalist conservative, is similar to this one. Yes, they are often right when they criticize sexism or racism in society. They are also more often than not wrong when it comes to their proposed solution. And what we're fighting against is not so much the diagnostic people make about prejudice, but an entire culture that has internalized these awful solutions as their inevitable outcome.
And the problem is that, particularly in our case, their solutions often goes against basic tenets of our worldview. If we take racism as a frame of reference, for example, you can say that within the "racism is a problem" crowd you have two distinct theories: one is the idea that racism is the result of a racist attitude, and that people of color are kept down because they can't progress in a society dominated by this attitude (that is, Culture shapes our material reality) and the other is the idea that these notions of racial inferiority could not exist if different races did not correspond to radically different socioeconomic circumstances, diverging from the norm in terms not only of capital, money and skill, but also of cultural and social capital (that is, material reality creates our Culture) and although most people occupy different positions in an entire spectrum between these two points instead of completely siding with one, the perspective you're more inclined towards can lead to some radically different conclusions from the people inclined towards the other. One means that you don't really have to change our material reality in order to erase cultural biases, so we should just move on towards eliminating those in the existing system. The other presupposes a radical change in this material reality, which means that we must re-organize our entire economy. So just because we start from similar points (e.g. Racism/Sexism/whatever-ism exist and suck) doesn't mean we arrive at the same conclusions.
And I'd say that this is where we diverge from them. Their complaints are not illegitimate, their methodology is. And that methodology is not only faulty, but it leads to some really vicious patterns of thinking and behavior. That is, if the way we interact with each other becomes the alpha and omega of our solution to prejudice, then as long as certain inequalities exist we'll continuously be rethinking our vocabulary, learning and re-learning how to act properly towards minorities, changing our clothes, tastes and behaviors in order to accomodate them, etc. These are all valid to a certain degree, but when it comes to fixing major inequalities (like, say, racial inequality) it will achieve nothing, and people will obviously react against it, as they already are. We can't always be having "conversations" and dramatic moments of "addressing the problem", as much as people seem to have fallen in love with this method.
And the sad thing is that this practice is not only mainstream, it is for the most part institutionalized, partly because certain powerful groups don't really want us questioning the impact of their economic practices in shaping our attitude towards identity. Some of us have made this connection in the late 60's and early 70's and then took it to the streets, and they know better than to allow us to arrive at the same conclusion again. And now they actively work towards stopping more radical groups from pushing Materialism back into the lexicon of social justice.
In general, for me, this is a question of who you want your enemies to be. If you want to fight Capital, to fight the owning elites, to change the anarchic and inherently unfair system they force upon us, and to abolish inequalities of all sort at its roots, you pick the materialist approach. But if all you want is to roll your eyes at your uncle's racist joke at the dinner table, to sneer at some peasant who isn't as clued up as you are on the 'proper' way to act, to jointly complain about some old white lady who stared too long at some black chick's hair, and to feel part of an exclusive group of people who get it, you choose the culturalist approach.
Sorry for the TL;DR I like ranting about this topic