I have been in law enforcement for approx. 10 years and I can tell you emphatically that many night shift officers use their dimmest modes very often. I would have recently purchased the PD36 TAC if they would have had a true low mode. The design of that light is good but they are definitely wrong if they think officers don't care about preserving night vision. Most of my career has been on nights including right now. Hopefully Fenix will consider true low modes for future duty use lights as well and not just EDC ones. 30 lumens is way too bright to preserve night vision. Having the ability with a light to turn on in the lowest mode and the brightest is very important to me, at least.
It completely depends on the light for me. In dual mode lights with a fairly bright HI output (250lm or more), between 20 and 80Lm works great for me as a low mode. In multi-mode lights with 3+ modes, there should absolutely be a <5Lm mode, but I dont really use those. Also if a light is not very bright to beginn with (<250lm), 5 lumens or less would make an appropriate low also. I actually consider the 20lm low on my Malkoff MD2 too dimm to be useful for most tasks, and wish is was closer to 50-70lm.