It's useful, because keeping an airflow over an inactive evaporator effectively warms it up fast (it's meant to happen, because evaporators are designed to exchange heat with the air in the most efficient way so that they can cool your car fast) so that water soon ceases condensating over it, and with it the ducting, keeping in mind that most car AC systems, when in good working order, have, on average, a 40F-41F air temperature at the vents (and no water condensation means that any leftover humidity immediately evaporates and get pushed out of the vents, and that's why you can feel the air coming from the vents get progressively humid as soon as you turn the AC system off but keeping the blower on).
Bacteria effectively thrives in an humid environment.
The time the airflow should be kept on is some minutes. There's not a precise time (it varies depending upon the heater unit, evaporator and ducting dimensions and arrangement). As a side note, keeping recirculation on during this period lets the evaporator cool down slower than with ambient air blowing over it, so that even if no refrigerant is circulating, you can still get cool air at the vents for some minutes, especially if you have an efficient AC system (one that can easily get, and maintain, the evaporator near the freezing point) and not one struggling to do its job. If you have a fixed displacement compressor, disengage the AC and keep the blower on when you hear/feel its clutch disengage: right after the clutch disengages, the evaporator is at its coldest; if you have a variable displacement internally controlled one, this doesn't apply because the evaporator is always at its coldest temperature; if you have a variable displacement externally controlled one, temporarily setting the temperature to a much higher value causes to evaporator to warm up due to less refrigerant reaching it.
However, if you have an habit of periodically cleaning the ducting and evaporator by spraying air-con cleaners, along with fitting new cabin air filters yourself when required, you don't have to worry too much about it.