With nearly every cell phone now having the ability to tether and the availability of wireless connectivity increasing having a stable connection or having the ability to increase stability is key. Also, imagine running an ubuntu server. The internet goes down. 3 people in the office have phones that can tether and 3 more have data cards. You plug them into the server and you have almost 6megs of shared bandwidth for the office. Also, with nic bonding you can easily use older hardware safely just by teaming up a couple of nics.
are you sure that your mobile network provider will allow you to accept incoming connections? (In france, you can't host a server on a mobile phone using mobile networks)
In the US providers allow incoming connections. However, you are right this is more for outgoing connections made. Mostly on the basis that any new devices connected would have many separate IP addresses.
If an office really needs internet access or has periods of high bandwidth. This method could easily be used to supplement bandwidth.