Thanks for the reply. I've read that in some of the forums.
Here's the latest email I received from Verizon on the question:
" I would recommend to only leave the device plugged in when you feel as if you will use it longer than the battery will last.
Also keep in mind that the when using the device as a Hotspot the battery life will be reduced.
Your MIFI 2200 has an advanced charger that will not over charge the wireless phone [sic MIFi], however please do not keep it plugged in for an unnecessary amount of time like hours when not in use."