The monthly rate hot spot is another income source for Verizon so they would naturally prefer everyone to use one. I went thru the same conversation with ATT, however eventually the sales guy agreed that there was no benefit to using a mi-fi versus my cell phone. At first he said it may tie up my ability to answer calls during use however I found the phone rings and I can answer, talk, and surf the web at the same time. Sometimes I charge my battery during use, but most of the time I don't. It uses the battery just slightly more than talking on my phone but not enough to be of concern.
