Well...
I'd say always, if you're subohming.
Reason being, SubOhm uses a lot more power and being tied to USB charging facilities is a pain...
However, I have three mods with internal batteries, all of which have far outlasted sets of reputable 18650s and all of which have been in daily use for a good couple of years now...
Admitedly two of them are for MTL tanks (3rd is dedicated to my RDAs) and are small enough (Curve 2 size) to be used mainly while driving these days, where I also have USB charging available but never had to use yet. But still... good internal-battery mods do last, especially for MTLers!