On a single target, a single-disease HB-based rotation has inferior threat. I tested it once using Kahorie, was pretty clear. The new IT Glyph would not change that in 3.3.
Other questions I'm not as conclusive (like GoD), so would have to double-check my instincts. I suspect FS and Oblit glyphs are better, and I know RS would be better.
For AOE though, single disease would do fine because HB isn't multiplied by 2 diseases, but by frost fever, and I'd expect GoD would be better in that situation than Glyph of FS or RS; but you'd be talking about a spec/glyph combo that's biased for AOE (ie. you'd pick the talent for more range and glyph of HB and GoD).