Dual booting on a HTPC kinda defeats the purpose, but then dual booting never made sense to me, even when that was the only option back in the early 2000 and I dual booted some Linux distros while studying, I never ended up using them because I does was what I needed and it simply worked better in general and that made it pointless for me to reboot all the time to play around.
The dual-booting is a needful because not even all Steam games play on SteamOS - in my current collection (one of the smaller ones), fewer than a half-dozen games from over two dozen are SteamOS-ready. (That has, in fact, been used in this very thread as a hammer to bash SteamOS.)
I'm not saying that the compatibility issue isn't a problem - it is, in fact, a rather large problem. However, it can be solved.
The bigger problem is the audio support - it defaults to HDMI. While HDMI support is mostly bog-standard (and that is true for nVidia GPUs as well as the not-supported-yet AMD GPUs), there are some odd ducks that don't. That means they will be requiring support for onboard sound or sound cards - even of the PCI Express sort. While even Debian wheezy supports (or can support) quite a few, SteamOS isn't quite up to Debian wheezy levels of audio support. (While both Debian and SteamOS detect my Creative Recon3D as an Intel High Definition Audio device (that is proper - it's detected in Windows as the same thing by default) - it cannot configure it as the HDMI-only support designed into PulseAudio by default gets in the way - and I haven't seen how to fix that yet.