If the atomizer gets a constant 5V, one factor that significantly affects its performance has been eliminated (it will draw the same current and generate the same heat every puff). You're not doing that with a passive resistor, whether a component or provided by the body of the device. Switch buck regulators can be 95% efficient, which means you gain vaping time (lower current draw).
Why would a device use CR2's? To be smaller than it otherwise would be, of course. Otherwise, everyone would use 25500 C cells.
Why not use LiFePO4 or LiMN cells that have no risk of thermal runaway? Because they have 25-33% lower energy density (LiFePO4 also has a lower base voltage), which means either less vaping time per cell or bigger cells. And just because they won't turn into a firebomb doesn't mean you don't want under-voltage protection, as running them way down can reduce their recharge cycle life from hundreds of recharges to dozens (or zero).
BTW, as far as I know there's no such thing as a protected LiFePO4. Li-Po(lymer), yes, but those are generally only available in flatpacks, and have the same thermal runaway problem as Li-Ion.
Give it *up*, man. There's nothing wrong with mechanical designs, it's just another set of design constraints to work around, a different set of tradeoffs. But they're not the be-all, end-all, either.
See, if you're such an expert on mechanical eCig's, you could have been helping this guy. Instead you decided to have a slapfight with me, so now I've got to do it, assuming he's even still paying attention:
A switchless design is one where the circuit is formed without an electronic switch, usually involving touching the positive end to an atomizer or battery fitting, or to a rod/nail/screw that contacts the atomizer fitting. Springs to keep things out of contact when not in use are generally involved. The advantage is that since the contact points are very large, the degradation that involves every cycle of the circuit being closed and opened is comparatively trivial. Since switches are the most likely point of failure in a battery mod, this gives them a high reliability factor.
Solderless mods take that a step farther, and do it all with no solder, just screw threads, springs, and plastic sleeves. They can be extremely tough physically, literally to the point of surviving being driven over by a truck with only superficial scratches. The downside is that they generally look like a pipe bomb or "potato masher" hand grenade.
--Dave
