Dynamic start-up circuit
Imagine the Vdd is brought up from 0V to nominal operating voltage very slowly relative to the RC time constant of Rs*Cs so that node B is essentially at Vdd potential at all times. MS1 could then never turn on. 

So this is possibly an unreliable start-up circuit because it depends on rapid application of Vdd. 

Keep in mind that on-chip resistors are probably (and preferably) relatively low resistance and on-chip capacitors are usually tens of pF or (preferably) less. For example, 100k and 10pF would have a time constant of only 1\$\mu\$s.