For those using a WiFi Ranger or other router capable of switching between input sources, a reason for switching the SIM from a 4620 to the USB290 is the MiFi cannot currently be connected to the router via the USB cable. The direct USB connection is faster than repeating the WiFi signal. You do need to purchase or make a SIM adapter since the micro SIM is too small to fit in the USB290.
Another reason is that while the 4620 MiFi unit provides a WiFi signal, it is nowhere as strong as that of even the basic WiFi Ranger. If you use the WiFi output of the device while driving or away from your RV, you may find the MiFi does not supply a useable signal level.
I wouldn't "throw the MiFi in the trash", but for some users there are reasons for switching to a USB device.