One of the lesser known wonderful things about current Fraz lights is that you can use either! (as you know). I have no qualms about using unprotected because max brightness will disappear when the battery gets low. I am good about not leaving it after use on really low, and even when I did leave it on overnight early on, the loss is minimal, and I use them regularly so the high brightness loss will become evident. Since I am in a pattern with them, it is easy to charge them every couple weeks or more, without ending up below 3.6V.
(Our dog flashlights (Emisar, Zebralight) have state of charge indicators, and we like to charge them at about 3.8V but wife fairly regularly takes hers lower. Son? Forget it. He is relegated to NimH.)
That said, when starting this rewarding hobby, I had flashlights that were suited for protected batteries and I was OK with that. Then Zebralight started chasing smallness and requiring unprotected, and Emisar requires unprotected, so I ended up not really using the protected 18650's.
Now I can use those protected cells in the Fraz; Great!
It gets me that a reviewer wrote about Fraz, complaining that they do not have low voltage protection baked in. Ignorance. The beauty of Fraz is the simplicity and the flash-in-the-pan reviewer wants it to be more complicated to suit his beliefs and low functioning users. I mean, if you are using a Fraz, you will see when it can't be run on high. Charge it. Or you will remember your pattern of charging.
If you are forgetful, or stressed, and you take an unprotected cell to null. Oh well, spend $10 or whatever and get another battery. Then keep track of things. Fraz is simple. Fraz works. Don't expect it to be something it is not.