[GadgetKen]

Some possible advantages of a MiFi device like that other than the obvious multi-user scenario you mentioned:
1) Using it for a wi-fi only device such as Sony Mylo for web browsing/Skype, an Apple Touch, a cheap netbook or a wi-fi enabled camera (either through the camera itself or a hybrid wi-fi/memory card.
2) The mi-fi device could be positioned in a building where there is optimal reception (upper floor or in a window) and the wi-fi part of mi-fi device would relay the internet connection to a location where the cellular signal would not normally reach (such as a lower floor).
3) Ability to share the wifi connection with visitors or clients without giving them physical access to your sim card. Advantage of using a prepaid sim card in this situation is that it would also be self-limiting by the amount of prepaid data remaining in the account. An alternative is that it could be limited with software to a certain amount of time and/or data for each users session.
I'm sure others can think of other benefits for the mi-fi device.