The best uses I have found for Holy nova, are the folowing...
1. Great for the Dance phase in Naxx againts Heigan the Unclean.
2. After Vortex in the Malygos fight when everyone gets dropped down in a huge stack, it's
perfect for quickly spamming a few novas to get yur grp topped off.
3. The way my guild does Auriaya in Ulduar is we stay stacked, its great there also for AoE heals.
IMO, if yur grp stays stacked on you, its much better then CoH, espesially glyphed. and im Disc,
so needless to say i use it alot.