Mechanical switches, as opposed to E-switches, aren't 'as susceptible to parasitic drain.
Unless there's something that bypasses the switch to draw current (like a back-lit switch?),
0 parasitic drain in fact. As for most flashlights that have a non-electronic rear switch (including the S2+).
But more importantly: this takes [failure of LED / driver electronics] out of the equation when the light is switched off. So effectively light off = PASSIVE storage tube for the battery. This safety aspect is one reason I usually prefer mechanical switches. I simply don't like the always-on / standby / "deep sleep" aspect for devices. Even less for 'simple' devices like flashlights.
More dangerous would be to leave these lights tailstanding for an extended period and/or unsupervised while on
at 100% setting (or perhaps even 40% ?). With 3A driver an S2+ may just overheat as your hand doesn't provide the heatsink + monitoring function in that case (and no thermal sensor in the driver).
So when tailstanding for extended period in a higher power level, check temperature from time to time.