What's funny is, you DO need a full-blown OS for this!
1. Many porn sites are really big on downloading custom-made plug-ins to use with them. The porn industry on the net has been one of the most aggressive in adopting and experimenting with new technologies. (They stand to make, or lose, a lot of money based on how user-friendly they can make their site while still locking down their content tightly enough so people can't just share paid user accounts, download and re-distribute their photos or video for free, etc. etc.)
2. Even something seemingly as "basic" as video streaming from popular sites like Hulu or YouTube may require a full-blown OS to accomplish it in the manner the user wishes to do it. EG. User may have a dual display configuration and wants all video to show on only the 2nd. display (or LCD/DLP projector that's attached to a second video port).
3. Voice chat sometimes gets more complicated than you'd think, too, because people want to use devices like bluetooth headsets. Even OS X has had numerous issues with people hearing pops/clicks or with pairing of specific makes and models of headsets in the past. A stripped down, basic, browser-centric OS aimed at netbooks isn't likely to support all of this very well.
These are just a few random examples, off the top of my head, but it illustrates the reason the most popular operating systems of today are all so huge and resource-intensive. They *have* to be, to make a lot of the things "easy" that people come up with to try to do, while trying to use what seems like a pretty straightforward application. A small, "mini OS" works best when you have control over the hardware, and you're only providing a limited set of capabilities (as you do with, say, the iPhone).
There are a lot of people in the world who only do a few things with a computer, even in today's world:
- Watch porn.
- Check their email.
- browse the web.
- Video streaming via Youtube, Hulu
- listen to music and sync their iPods
- documents and spreadsheets
- organise and edit some photos.
- MSN, Skype, etc.
That covers most tasks for most people. You really don't even need a complex OS like OS X or Windows to do these basic tasks, which is what's funny about OSes getting more complex and feature-filled.