Holy priests under appreciated

[Mercy]

Metrics are part of this game where people compete.

Of course. However, there are more metrics than HPS and gearscore. Even in just skada/recount itself, there are other tools that are useful to gauge how well a healer is performing. The community at large doesn't recognize or use these metrics, though. Either because they are ignorant of them, or think they are irrelevant for whatever reason. There's no sense in trying to argue or convince someone already set in their ways. I was never successful with this. Where I found my success is when I blew all of the other healers out of the water with the metrics these people understand.

Thankfully, I don't PUG, but not everyone is so fortunate.

and other fights where I am almost near the holy paladin.

A lot of people forget to exclude Judgment of Light from HPS calculations, as it's free healing that usually a holydin will whine and cry about because it's the only way most of them can do any good on the meter.

Also, not only the healing but the mechanics of the class help a lot in some encounters. For example, Body & Soul is awesome in so many fights, including LK... Also, PoM is so useful in some parts of fights (Sindragosa while waiting for Unchained to explode, Rotface when it spits to ranged that can't avoid it...).
This community has a tendency to deem some specs useless without thinking why.

Guardian Spirit is very powerful too, if you use it properly. If you use it for its healing increase instead of a death prevention, you'll find you'll have it available a LOT (with the glyph). Don't be afraid to stack it with other tank CDs, or on people who need emergency healing.
One could also mention the power of a HPriest's group healing. Which leads me onto the next point;

The only reason you don't see as many priests here as on retail is because over the years of minmaxing wotlk, it got gradually eclipsed by Rshamans (and rightly so..).

The rest of your post is on point, but I'm not so sure I agree with this point. Chain Heal may be a smart heal, but you still have control over where Prayer of Healing goes, Prayer of Healing is more powerful if it heals all the targets it hits, and it has greater range. The two are a bit different, and I think different enough for each of them to remain useful. So the comment about "rightly so", I'll disagree with.

That said, however, I would say I also feel that the HPriest wasn't just eclipsed by Resto Shammies. In the 30% buff environment, HPriests are eclipsed by essentially all of the other healers because of how easy it is to heal in the 30% buff environment. Why would you play HPriest when you can effectively play all of the other healers with 1 button for the most part? Lazy people smh... but it makes sense. Outside of this environment though, HPriest competes very VERY well. HPriest is still very useful in a 30% buff environment, it still performs well. The difference is mainly in that you can get away much more easily without having those Prayer of Healing or the versatility of the HPriest when the risk of death is much lower.

[Zeaconn]

Rshams also get ancestral awakening which is another smart(er) heal, aswell as one of the best 1GCD spell in the game: earth shield. Ancestral awakening + chain heal + 4t10/earthliving make it very hard for a Rsham to not perform at least decently imo. Not to mention that they can start functionning on a budget gear.
And in the hands of a smart player, they're so flexible in terms of heal roles that you can basically take 3 of those in your 25 man group with no real downside.

I agree that 30% kills HPriest, Rshams in 30% buff are basically just safety nets for raid fails anyway.
I also agree that in 0% and in the hand of a competent player, HPriest's strengths (strong as **** tools and massive bursts of HPS when needed) make them very valuable.
The difference in my opinion is that they are not fitted to all fights (I know that not a reasonable thing to ask but well.. Rsham are always fit) and can lag behind if they don't get gear, unlike hpal/rsham

[IcarNS]

Guardian Spirit is very powerful too, if you use it properly. If you use it for its healing increase instead of a death prevention, you'll find you'll have it available a LOT (with the glyph). Don't be afraid to stack it with other tank CDs, or on people who need emergency healing.
One could also mention the power of a HPriest's group healing. Which leads me onto the next point;

Yes, that one too. I also use the glyph and not only for people but it is very useful with VD when Hero is casted. Massive healing to the boss. I must say that I rather use this to increase healing because of its lower CD other than throwing it only to avoid death as it allows you more than one cast per fight.

By the way, now that we're talking about it: any other smart situation to use it? Maybe inside Frostmourne?

[Mercy]

Situations where you need to emergency nuke heal a single target it's always good for. On Lich King, you should have it for every-other Soul Reaper. During Frostmourne, if someone is really low, then yeah you can most likely use it there since it's available so often. Other situations would be like diseases on Rotface, if you don't dispel them out, Guardian Spirit can help offset the healing reduction on one that is in risk of dying otherwise. If you think about it and pay attention, you can find all kinds of uses for it!