the problem, for me, with internal batteries is the battery life, I will kill an internal battery mod in 2 - 3 hours

... I use dual 3000mah batteries and eat through them in 6 - 8 hours
the other issue is that yeah, once the battery goes, the mod is pretty much useless
internal batteries are nice as they do tend to be smaller sized but that also usually means less battery life than an external battery mod
I have 2 or 3 that just won't hold their charge anymore...one of those goes wonky and the coil resistance on it jumps all around the place so not sure if there's also a 510 connection issue or if it's due to the battery issue...
that's good point and I have done that before but as David mentioned, you're stuck being connected to something and it doesn't really have much umph behind it to ramp up and heat the atty right, so I usually end up with a weaker, unsatisfying vape...much like you'd get when it's time to change the coil and the batteries are nearly dead....