My first thought was similar - because I was picturing actively self-replicating malware, and you're right - if you don't want that on your own network you shouldn't be opening up everyone else's network to it. But I think the point is just to open up pockets inside your trusted network for less-trusted machines. It's not that you think they have viruses, it's that you just don't want them to have access to the rest of your internals. Most places have an entirely separate network for guests, but if this was reliable and robust enough, maybe it would be a nice alternative.