Can only talk about the MiFi which I had through Virigin Mobile which is basically the VZW one. I always had it plug in when using it usually and always had it on as well. While I wouldn't expect it to last forever I didn't really see a drop-off in battery production.
Basically I think its going to kill the battery over time a little faster but nothing more than charging it and draining it all the time. Only thing that might hurt it is if the system doesn't have good ventilation while charging as it could overheat from charging and being used.
*Note I am not a hardware expert just talking from my experience with my Virgin Mobile MiFi
The guy at Best Buy told me that charging my verizon 4GLTE hotspot device was bad for the battery. Is that true?
We are not aware of any issues affecting batteries on any 4G devices while being charged. For more information about your device battery please see the links below: