I like how Dr. Laura puts it -- If you're not going to do the actual work of raising children, why have them? How would your employer feel if you only put in a few hours at night and on weekends? Would you be considered a valuable employee? Would you be considered full-time, especially if you sent in a substitute to take your place? No one can take the place of a loving, dedicated mother.
Others have said, but I want to say it myself. A mother works 24 hours a day, seven days a week, 365 days a year. My adult children STILL call me to, as we call it, "plug in" to Mom for emotional sustenance. Mothering is hard, continuous work, for absolutely no financial remuneration.
And I LOVE IT!