3-Kam walis are less likely to be abuse from their husbands, since their employer (women) would alway have sympathy with them(being a woman them self)
My aunt was telling me that when her maasi was getting a proposal - the prospective husband's first question was how many homes did she clean. Once he knew how much she made/expected to make did he propose to her. In many families, the maasi/kaam wali was the sole source of income.