Current wisdom is IMR batteries are preferred for ALL applications over protected batteries, including in mechanicals.
Resources: http://www.e-cigarette-forum.com/forum/ecf-library/129569-rechargeable-batteries.html; Mods, Batteries, & Safe Vaping; http://www.e-cigarette-forum.com/forum/battery-issues/254690-protected-batteries-vs-imr-safety-5.html; Mechanical Mod Proper Usage Guide
It has nearly gotten to the point where there is no reason to purchase/use protected Li-Ions for any application in today's world of vaping. The inherent safer chemistry of the current IMR and hybrid batteries make them a better choice for use in both mechanical and regulated mods.
The only reason NCR/ICR Li-Ion batteries can be used at all is because of the protective circuits that are added to them. Protected circuits can and do fail. When they do, these L-Ion batteries are then just the volatile (flamable) chemistry which can vent flames and be explosive when they fail.
IMR and hybrid batteries are a safer, non-volatile battery chemistry. They can also vent and go into thermal runaway; however the results will be less dramatic and without flames. Their stress and heat tolerances are higher than the other battery chemistries. Therefore they are the safer choice for all applications.
"In a mechanical mod with a solid metal piston for a switch and 0 wires, your weak link is the battery... this is not a link you want to break while it's in close proximity to your face or anything you value more than those clouds of vapor which will dissipate a little faster than your checking account if you over tax a battery and create a little time bomb.
Yes, to the people who will say "it's not that easy to burn out an IMR!" you're right, under normal circumstances it's not. But these batteries are NOT intended for what we're using them for. In fact, after recent conversations with Panasonic/Sanyo, Sony, and Samsung, they don't even like the fact that we're USING these batteries because they're not intended for single cell, unprotected use in any device. The fact that they're available can be attributed to modders of flashlights, pen lasers and bicycle electronics. A demand formed around those markets and it was filled by various folks, ecigs came along and the demand skyrocketed." Resource: Reddit.com