Why?
Not being flippant. People say that you shouldn't charge via USB, why not? I have owned 3 different Mods in my time, one was an internal LiPo, the other two have 18650s. Both manuals from the 18650 mods actively encourage USB charging yet everyone on forums discourages it. From my limited understanding of the technology involved there is no real reason to not charge via the USB. I have done it many times and nothing bad has happened.
My worry would be that because each cell is essentially joined together in the mod (in series I presume) how is the mod knowing exactly what charge in each cell? ... is it just taking an overall voltage reading for the 'pack' or is it able to read and charge each cell to exactly 4.2v? In my experience the 'battery life' display on most mods is pretty inaccurate, so how would I even know if it was doing it's job?
the cells we used aren't 'matched' so never drain exactly the same, what happens when the cells run down unevenly (which they will so some degree) are any of them being under charged or over discharged as over time that will just exacerbate the issue even more.
There's plenty of mods that don't even appear to be able to fire accurately at the power you have them set at, so I'm not sure I would trust them to accurately charge my batteries.
..... I wouldn't trust a Smok mod to do it that's for sure, or any mod that looks like all the R&D went into flashing lights rather than safety.
.. that's what it boils down to, whether you trust the electronics in the device can do the job properly. ... if you do, then that's fine.
You could always pull the cells after a few weeks of charging them via USB and measure the voltage fully charged and see how good of a job the mod it doing I guess. if they are all at 4.2v after charging (or at the same voltage 4.15, or whatever) then happy days I guess.
a lot of mods will charge at 2a too, which isn't great for say 30Q's who's standard recommended rate is 1.5v. ... and it does worry me how hot some mods get when you charge them too.
... and you are creating a lot of wear and stress on that crappy little USB port that is only held to the board by a few small solder joints every time you use it because they are just 'floating' in a hole in the mod usually.
I'd do it occasionally and I don't think anything disastrous is 'likely' to happen.... but it doesn't seem like the best option to me.