Spivak doesn't say that sexist practices in poor nations should be excused because they are part of their "culture" or anything like that. The main point she is making is basically that oppressed women in the Third World become a kind of political football between nationalist men and liberal Western imperialists (including liberal feminists). At the risk of oversimplification, I say that Third-World nationalism often mobilizes the idea of the nation that masks class & gender oppressions (with a partial exception of revolutionary nationalism inflected by Marxism); Western liberalism smugly presents itself as "enlightened" in contrast to "backward cultures" and uses this contrast as an excuse for denying self-government to poor nations (in the process Western liberalism also turns a blind eye to the fact that its own imperialism has helped to perpetuate and often to intensify the very backwardness that it pretends to deplore [e.g., Western liberal support for fundamentalist "freedom fighters" in Afghanistan]). Neither nationalism nor Western liberalism serves poor women's interests (first of all, because neither abolishes material conditions that give rise to the oppression of poor women). So far, so good. Spivak, however, used to argue that trapped between the rock and hard places "the subaltern [e.g. poor women in poor nations] cannot speak" (I don't know if she's still committed to this statement). I'd say that Marxism-Feminism (coupled with fight against imperialism, it goes without saying) is necessary to avoid the problem that Spivak points out.
Yoshie