Windows really needs to have more native multi-monitor options, a lot of enthusiasts use multiple screens these days, letting you change the 'numbered' priority of your displays would be a good start, my TV is display number '1' which is bloody pain and it's not easy to change.
I agree. However, with the move to tablets and stuff that MS is doing, I don't think that's a priority for them. They seem to care more about one screen, than power users at their desks, or with elaborate home theater setups.
The best method right now to deal with a multi monitor TV + desktop scenario in Windows is to use UltraMon.
I had a setup as follows:
Monitor 1 + Monitor 2 | PC | <---Several feet---> HDTV
All of them were hooked up to my PC at the same time (both monitors over DVI, and the HDTV over HDMI). I used UltraMon to create two profiles, one where Monitor 1 + Mointor 2 were used, and another where Mointor 2 and HDTV were used. I then had those profiles saved as shortcuts, which I pinned to the start menu. So to get the TV interface was simply a matter of pressing Start --> "TV Mode".
It worked really well.
The only issue I had was sound, but I found a real lightweight program called 'Win7AudioSwitcher', which had a cool function that remembered what your default audio device was in relation to your primary monitor. So I taught it to remember my PC speakers for Monitor 1 as primary, and HDMI audio when the HDTV was my primary. It worked perfectly.
Basically, what I'm getting at is that it's probably not too difficult to put basic monitor and audio switching as I explained above into Big Picture Mode. If it can be done with third party applications, I can't see why Valve can't have it built into Steam. With a bit of configuring in the options, you could tell it which monitor to go to and what audio device to switch to when you press that 'Big Picture' button, and then Steam knows to revert back to the old one when you go back to the desktop.