Yes, it's better to not charge by USB. Gosh, when I first saw some of those mods, I thought the USB port might be useful for charging my
phone. I did not realize.

Also, trying to charge by USB will take forever, so yes, unless you can last a whole day on two batteries, and charge while you're asleep.... Well, chargers are really inexpensive (I have the latest generation Nitecore 2 bay and it charges well in a couple of hours.) I started out with 4 batteries with a 2 battery mod, now I have 8 so I can rotate them and let them rest. But, it's always great to have a spare set of batteries (in that handy, plastic carrying case so nothing can get at them) for out and about, in case I am not paying attention much to my batteries and if it needs to be switched or not.
Batteries/charger are probably going to be your LEAST expense, after upfront costs. If you take good care of them, they will last longer.
Anna