...wondering why I would want to share a network with a bunch of devices I had no reason to trust. I know I can isolate them on my home network...Unfortunately, it's actually quite difficult to do that properly with a typical broadband/WiFi box as supplied by most ISPs (at least here in the UK) by default. You need to step up to business-grade gear in most cases, which is both more expensive and beyond the technical knowledge of most consumers, and even then some otherwise respectable devices are regrettably lacking in flexibility when it comes to VLAN configuration and the like. I'm looking forward to the day when standard home user boxes allow you to do things like isolating certain physical ports or wireless networks by default, so hopefully running untrusted devices on independent networks becomes routine.
Even then, there's still the problem that if you want to watch some sort of streaming content off the Internet, whichever device(s) have that external connectivity could also pose a privacy risk if they have access to whichever devices are acting as home media servers. DLNA seems like a step in the right direction for connecting up home media devices, but I haven't seen much evidence of a robust security model being implemented so far.