Unprotected batteries in general is a bad idea for the life of the battery, they need some type of protection so they last a lot longer. For example from google, I'm sure the same applies to 18350 batteries since they're both quality li-ion batteriesSo I'm not crazy! Every saber I've installed so far has used wired protected 21700 or 18650 batteries. Some have been more difficult to cram together than others, but it always felt like the chassis were designed to allow for wired batteries exclusively. I'm also seeing some installers using unprotected 18650s in MK-1s with crystal chambers, but I've not seen that recommended anywhere. Have you attempted it?
Most modern 18650 batteries have a typical cycle life of 300 - 500 (charge, discharge cycles). When in high-amp or high-drain situations, this can decrease substantially to 200 cycles. If you go over the maximum discharge current limit (A) you can decrease the cycle life all the way down to 50 cycles.
Those protection pcbs keep them within a tolerable range to maximize their lifespan