My friend pointed out that only using the 18650s to 3.6-3.7 volts a day will greatly increase the 18650's lifespan instead of draining them all the way to 3.3 volts. I didn't think about that but he's right.
Yeah, but as cheap as quality 18650 batteries are these days, I'd MUCH rather have the longer run-time than the increased life-span.
Seriously, it's pennies in savings, at most maybe a dollar a year per battery if you "baby" your 18650s and don't drain them as deeply. False economy, really.
And there's no doubt in my mind that ~19 watt hours from dual 2600mAh 18650s in serial is superior to ~10 watt hours for a 900mAh 3S LiPo, and well worth the drop to 133 watts max power for a lot of folks, as is the ease of swapping out discharged for fresh without being tethered to a USB cable. Of course, the cost of this is a potentially larger mod, but this first round of LiPo-powered DNA200 devices are almost all ludicrously over-sized already (Vaporshark and maybe hcigar models being the exceptions), so even that's sort of a non-issue.