Is it practical to boil drinking water in a pressure cooker? At my home and relative's homes I've never seen anyone using a pressure cooker to boil water (to make it safe for drinking). They just take an ordinary steel vessel, fill it with tap water, add some salt and jeera seeds, cover the vessel top partially with a lid and let the water reach boiling point.  

I just realized that it might be far more efficient to boil water in a pressure cooker until the first whistle sounds, since it's a closed environment that would lose less energy to the environment. It may boil the water faster (less cooking gas consumption), boil it to a higher temperature (greater variety of bacteria dead) and perhaps lose less water to evaporation. Is this practical or are there downsides to this technique because of which people dont use it?
Overall it will make little if any difference. 

The pressure cooker won't reach 100&degC; noticeably quicker than a normal pan with a close fitting lid, and the pressure cooker is made of thicker metal which will take more energy to heat. Simply closing the lid would help quite a bit. 

If you want to hold it at a boil the sealed lid of a pressure cooker would help, except you can't really monitor the temperature. Waiting until it whistles will mean it has been boiling for long enough to produce enough pressure for a whistle, but that's still a measure of pressure, not time. 

There are a few species killed by the temperatures achieved in a pressure cooker, and not at 100&degC; but my understanding is that these aren't the pathogens that tend to contaminate drinking water.

As you're flavouring the water as well, you may have to adapt the quantities of seeds if you use a pressure cooker, because the flavour will extract faster.