I have an Ali'i, which is a bottom feed device which can use either 3.7 18650's or two CR1213's at 3.0v, which gives you 6v.
So far, I've only vaped using 3.7, but I have tried another device with the two stacked 3v batteries, for 6v. vapping.
It is a MUCH warmer vap, which to some may replicate the sensation of smoking an analog more accurately. Also, with 6v, you take a VERY quick drag, and you're good to go! None of this loooooong, sloooow drag, then inhale, then exhale.
With 6v, it's a drag which is much shorter and quicker, therefore (again) simulating the sensation of smoking a little more accurately.
You are correct, however, that while the 18650 battery will last me all day with the Ali'i, a pair of CR123's will (I hear) last about five hours.
The guy who makes the Ali'i, his name is Ryan, told me once that he generally uses 3.7v when out and about, but will switch to 6v's on his Ali'i when he gets home.
Kind of makes sense!