Thanks-- very useful discussion. Just to add a Canadian perspective (whatever that is)- essentially in ice and alpine climbing the use of half-ropes provides redundancy, as the pro is often suspect, and here in the Rockies chops from falling rock are a real concern. Standard practice is to alternate clipping in ropes to closely spaced pro above the belayer, running it out more as you proceed. W/ regard to cruxes, I again place multiple closely spaced pieces and alternately clip in. I don't see any particular advantage to using twin ropes (perhaps a slight weight savings?), and have never encountered a situation where it is necessary or desirable to clip both half-rope strands into the same piece.
Additionally, a recent 20' leader fall on ice on a standard draw (3rd piece above belay-- screw held) scared me-- went out and bought a bunch more screamers. Basically, whatever you can do to reduce force on suspect pro is desirable. Unfortunately screamers weigh more, but they do add peace of mind.
The only time I use a single half-rope is in situations where 1) I'm assured there won't be any rock or ice climbing of significance (e.g. some classic mountaineering glacier routes), and/or 2) there won't be any full-rope rappels. These situations are rare in the Rockies, so usually we carry two. When we first moved here we were in multiple situations where we tried to save weight by carrying one rope and regretted it. Sometimes if only (2) is relevant we'll use a half-rope plus carry a 7mm zip line to save weight, but this is rare at least for routes we do.