The MBR1000 is a nice unit as it allows you, as many have mentioned before, to share the connection load (load balance) between several different cards. There are two USB ports, an Expresscard slot, and an Ethernet port if I remember correctly. This allows you to use up 4 different sources of connection, which means if you for some reason had a ridiculous amount of money and were able to afford 4 different cell carriers, you would have an incredibly redundant system and would probably rarely run out of coverage.
I will mention a few problems however. The router wants to prefer the Ethernet connection if you have anything plugged in to it. So if you have something there and something in a USB port for example, you'll have to manually switch between the two. It also has some problems in actually switching the connections. Basically there are two settings you can use. You can share the load between everything, or you can have it switch to another card when one goes out. However, the latter setting requires that you set up a ping time, which means that the card pretty much has to be getting absolutely no information for like a minute before it will switch (the unit will ping the internet over the card for the specified time and if it does not receive a response, it switches the connection if available). Most of the time you can't really function on the internet well before you hit that point, so if you use that setting you will probably want to monitor the connections and manually switch between cards (assuming you have more than one) to whichever card has the highest connection signal.