That's the first step to unionizing: recognizing that you can act solidaristically to fight the unfair treatment and it's the first step domestics are going to have to take. Compare this to Walmart workers where there have already been chain-wide attempts to unionize. Domestics find themselves in quite another position when compared to Walmart workers, correct? The problems of organizing workers in different occupations which are structured differently because of the nature of the work is well-known. So, I think your concerns as to why she doesn't mention unionization are misplaced. I imagine that, were you to write Ehrenreich, she'd say "Of course, I think domestics ought to be unionized."
Her answer to the problem isn't to have everyone stop using domestics, remember. She is speaking _only_ to feminists who hire domestics. She thinks it's a bad idea because she thinks it drives a wedge between professional/managerial women who are feminists and women who do manual labor. I don't think this is the problem she makes it out to be, but I do agree with her that feminists have had to deal with the fact that those who speak and write about feminist issues have tended to reflect the interests and concerns of professional/managerial women, often excluding the voices of lesbians, poor women, women who do manual labor, women of color in the US, and third world women.
Kelley