The placebo pill week was only built into the monthly pill pack because they thought that women would think the Pill was ‘unnatural’ if they stopped getting their periods altogether. Fact is though, since the Pill prevents ovulation, it also keeps the uterine lining from building up. Therefore, you don't really need to menstruate each month when you're taking it. Plus, your ‘period’ while on the Pill is really just what experts call ‘breakthrough bleeding’ – light bleeding of the uterus in between actual menstrual cycles.

Reference: http://www.arhp.org/Publications-and-Resources/Patient-Resources/fact-sheets/Understanding-Menstrual-Suppression/