Lol, I am not sure where you got that information from. Actually most lights that use optics do in fact have a lens in front of them to protect the optic and increase water resistance. A short list of examples of companies that use a glass, sapphire, or UCLp lenses over their optics are: Surefire, Malkoff, Oveready, Streamlight, Inova, Armytek, Emisar, Manker, Convoy, etc. Not to mention most custom makers lights as well.
The losses from a good quality AR coated lens are less then 1%. Even a low quality coated lens is generally less then 3%.
It should be noted that every scratch on an optic lowers its efficiency and increases transmission losses. Considering the average acrylic optic already has losses of more then 15% it is wise to protect it to prevent further performance losses.
Acrylic optics scratch very easily in my experience and it is not very hard at all to put a deep scratch in one that will show in the beam(even if it is only easily seen on the white wall). It should also be noted the acrylic can be damaged and/or discoloured by many chemicals including most insect repellents and gasoline. A quality scratch resistant coated glass or sapphire lens on the other hand can be very scratch resistant and if well engineered and properly protected by a bezel is very shock and shatter resistant as well. It is also impervious to most chemicals. Personally, I would much rather have a flashlight with its optic protected by a quality scratch and AR coated glass lens.
Even if the performance losses that the scratches, melting, and discoloration cause on an acrylic lens
are not easily noticeable in normal use, I like to keep my things in good condition and it's nice not to have to worry about it scratching and/or being damaged so easily. Plus, replacement lenses are cheap and easy to come by in almost any size unlike some optics.