So you could prove that in any of the Ulduar encounters you use more Holy Nova than Flash Heal and PoH?
I'm gonna reveal you the secret den of Hogger's Father if you do that.
That's not the issue here. It's about which glyph is more beneficial to your raid. Being that we can easily swap glyphs between fights now, it's just lazy to pick 3 glyphs and never swap them. Sometimes mana is not an issue at all and that makes the FH glyph worthless [for that fight]. Sometimes, well basically 95% of the time, the PoH HoT isn't going to save anyone's life. On the other hand, being able to hit your party with HN, for about 1/2 the healing done as PoH, while on the move will save someone's life. A lot of the time a full PoH will be a lot of overheal and a HN is amazing for topping your ranged party off while saving your CoH for other players.
I allways roll with GS(because I just love it. Its great to pop GS to save a raid member, or to put it on the MT and then being able to focus on keeping someone else alive without having to worry that hes gonna get killed.), CoH(because one more person getting CoH is one more person less likely to die :P), and PoH(because it just feels nice to have an extra hot ticking off while you PoH the next group )
I myself never used the HN glyph, but another priest did on mimiron and it works really nice, 3-4.5k heals, which you might think it way to much for an instantly spammable heal. the downside with HN is its range and mana cost, its easy when youre on the move to have one person get out of range for a sec and miss one, and spamming it will burn alot of mana. Even so i find it very intresting that someone finaly thought of a use for HN(Ive said since i saw the glyph that it could be good, but never botherd to test it :P).
It does however give you the advantage of a CoH, instant, but it doesent have a CD, which can make it superior to CoH if used right, and is very usuable in fights like mimiron where you have to be on the move quite alot. If you group up and mark each priest in that group and tell people to stay around him/her, HN is a very effective heal.
Also, since this spell is aoe its not affected by LoS either, which kinda tempts me to try it in pvp :P, specialy if the new glyph is 20% more dmg(if youre already good at kiting people around boxes/pillars, this could prove to be a dangerous spell if used right) and im very eager acculy to try it out