Yeah if a device has built in charging why do people feel you should not use it?
The main concern is that not all mods are build equal and there's been a few with crappy charging circuits.
I have multiple Eleaf mods, and they all charge just fine. The 50W and 100W had issues I believe, but every other model have been good or even stellar.
Another reason is that many mods charge at 1Amp, which is a "rapid" charge approach, while a charger will have the circuits to monitor the battery's charging rate and adjust.
Both mods and chargers should all flip to trickle charge once the battery is full, but batteries do get better life when charged slowly (at the 500mA or similar rate)
Most chargers will have a fast or slow mode. Either a switch, or by number of batteries being charges at the same time.
But with a mod, it's not that hard to prevent the 1A fast charge, just use a 500mA wall charger or use a computer's USB2.0 port.
Just do the charging overnight.