For now the chances of finding a third GoTenna user is quite low. In the future though this could be an alternative private Internet. A BitTorrent client could be a hit.
I think you are over estimating the bandwidth involved. They don't state much about the wireless protocol, but I am betting you don't want to send much more than a text message to get that range.
Which means that, no, they probably can't do seriously large amounts of data, but a picture (and definitely texting) can be sent through radio waves without much data retention issues.