Re: What is "parasitic drain" and is it important ?
If the parasitic drain is only a few microamps, I wouldn't sweat it... the cell probably self discharges more than that. If it's hundreds of microamps or into the milliamp range, then it's more of a problem and you'll have to remove the battery or unscrew the tailcap when not using the light so you don't have a dead battery down the road. Chances are unless it's a really cheap piece o'junk light, it's not going to have that much parasitic drain anyway.
Some forms of parasitic drain are caused by the circuitry itself... anything in the circuit that draws power constantly, such as a pull-up resistor that has something "pulling down" on it for example, or a filter capacitor that has some leakage current, or any contaminants on the circuit board. The microcontroller may draw a small amount of power as well, depending on whether the firmware implements the sleep mode properly or not. If the UI is handled with interrupts, the controller can go into a full sleep where the clock stops completely and no circuits are switching, this results in virtually no parasitic drain from the microcontroller. If it's handled with polling, it still has to poll so it can sense when the button is pressed--this requires some sort of clock to be running, usually at a reduced frequency when in standby... sometimes a low frequency watchdog timer is used to allow the controller to sleep and only awaken a few times a second to poll the switch.
It's likely that most, if not all lights with soft switches use interrupts, though it's possible some cheap lights may still poll. If there's a lockout feature in the firmware, ideally it should go into a "deeper" sleep mode if one exists compared to normal off/standby. Of course, physically disconnecting the battery by unscrewing the tailcap eliminates all parasitic drain within the light... then you only have cell self discharge to worry about.