Protection for power-off case with active input
There are probably ESD clamp diodes from the input to the GND rail and power rail of each IC. So the result would have a partially or fully powered IC from the current being provided by Vin.


  
    since Vin should be larger than Vth of the MOSFET in order for the switch to be on.
  


This remark doesn't make sense. You are either mistaking the gate drive voltage (the voltage between gate and source pin) for the voltage between the drain and source pins, or something else. Furthermore, for switching Vth doesn't matter becuase Vth is just when the MOSFET barely starts to turn on. When you use a MOSFET as a switch what you care about is the gate-source voltage at which the MOSFET fully turns on which would be the gate-source voltage used to obtain the rated RDson in the datasheet (or you can look at the I-V curves and calculate the RDson at any given point from the slope). You need to review how MOSFETs work in general. If you provided a schematic of what you had in mind then I wouldn't have to be so vague.