I think it really comes down the roles each healer fills. This varies by raid but for the purposes of our conversation (and based on my opinion/experience naturally) I see healing falling into categories:

1) Proactive Offensive Healing/Shielding - This is where your paladins and disc priests fall. Their job is pretty straightforward, provide steady HPS (or damage reduction by way of absorbs) to the tank(s) throughout the encounter. Once upon a time back when LB stacking was viable, we were here.
2) Reactive Offensive Healing - This is where your resto shamans and even holy priests would fall. When the damage hits they provide the burst to bring people out of the danger area. Prior to WG cooldown and even after with 4t8, we were here.

So where does this leave us now?

You could argue that our presence on the healing meters is a result of sniping and/or just spamming and catching effective healing. I would respectfully disagree (for the most part), but again I feel like this comes down to style. Most end-tier raiding druids are playing the versatile middle ground which I've always referred to as "Proactive Defensive Healing". It means that a strong resto druid is going to see the holes the other healers can't cover and fill them. The other healers aren't bad, this is a naturally occurring hole that our tools give us the opportunity to fill. To do this the idea is simple, yet harder to execute. You need to know the damage that is going to happen before it occurs. Your HoTs need to be in place to meet that damage as it happens. I feel like this is a major failing point for new druids.

Our healing prevents that sub 30% bracket if said druid has a definitive understanding of the fight mechanics. I won't disagree that some sniping is inevitable with this style. However, our job is to mitigate damage and keep people out of that danger area and doing that results in a fairly aggressive healing style when done correctly. This is the reason you will see some druids top meters regardless, even if the fight is not a Val'kyr Twins type scenario. They see the damage before it happens and have already provided the proactive defense to meet it. So, that when it happens, those same druids are pulling out tools like Swiftmend/Nourish as opposed to casting Rejuvenation/Regrowth (because those people that took damage had HoTs on them before it even hit).

It is less tangible in terms of "life-saving" measure compared to what pretty much every other class does. But to call it less valuable seems unfair to me. I will concede that it is not hard to be a mediocre or even "good" druid. Being an exceptional druid requires anticipation, awareness, and a very strong fundamental understanding of the proper tool for the job, both before and after the damage has occurred.

I see you already implemented a few things from my PM earlier. I see a few things that you could add if you see fit.

Unlike our other "over time" effects like rejuvenation, Abolish Poison ticks/removes the poison on application. Just a minor thing, but I think it is worth mentioning and adding into the spell description.

Wild Growth: You may also want to add the targeting mechanic of WG. You mentioned it may not always hit your target, but also you CAN cast it on a hostile target. A quick example is the Volatile Ooze on the Prof. Putricide encounter. I target the ooze and hit WG approx 1 second before it explodes on it's target. It helps heal up the Ooze Eruption damage quicker.

[Revitalizing Skyflare Diamond] is a higher throughput upgrade on a 5x1 spec with 4T9 than [Ember Skyflare Diamond]. Also, when tank healing, with the high crit chance on nourish, this also provides a higher HPS upgrade for both LS proc's from Nourish and LS critical, as these heals from LS would stack exponentially with the meta. It may be worth mentioning.

None of these are do or die changes, just minor suggestions for additions to your post.

I will stay out of the Rapid Rejuvenation discussion, but also throw in this about the regular Rejuvenation glyph; you can't really base how its effectiveness works on farm content. It see's it's glory of AoE fights like twins not on farm, but during progression content, when your SP is lower, player health pools are lower, Resto Shamans heal tanks more as their gear has them take more damage, and factors of the same concept leave the raid (typically your assignment) with less heals in general. The actually proc's of this glyph will reduce as your gear up to an encounter, rendering it less useful. Sindragosa progression, both Normal and Heroic, you may notice this glyph to really get it's usefulness.

The past 4 months of raiding, healing hasn't been much of an issue. Heroic Twins progression (for those of you who progressed) was the last HPS speed bump. None of the ICC fights so far have been close to healing intense, so it is really hard to judge accurately with current parses how "useful" your Rapid Rejuve or Regular Rejuve glyph is until we get into difficult healing content. Maybe this is just how I see content now, but I haven't worried about my healing effectiveness since Firefighter/heroic twins.

I need to disagree with you here. I've tried RR a few times and found that it is least useful when I have strict group assignments either because of another tree or other healer. In a 2 to 2.5 group cover model I can maintain Rejuv on all targets while still having GCDs available to handle targeted burst. With RR, I end up in a position where I am gcd locked to be proactive, so I instead need to switch to a reactive model. With current fights I find the proactive models is more effective, letting me cover the group wide damage (from many folks in cold flame, DnD, fester aura) and the individual spikes (shadow bolt, bone spike, vile gas).

In 10 man I could get away with reactive healing, and the glyph really shined. But for 25 man it just wasn't working.

This is very true, and is part of why this isn't an itemization question; it's a better play question. Player A and B are both wasting spellpower (and crit, any other throughput stat), but only player B is wasting haste. However, if player B had 100 haste rating, he'd have been wasting that, or he had 1000 rating, same thing. Percentage based, not an absolute number. Because they're all throughput stats they're all interconnected. Since we're not trading spellpower for haste in general, it's not a matter of 'what could I better spent those haste itemization points on', it's a matter of 'how far from the optimal play nirvana that we tend to assume am I?'

To put it another way. Every time we overheal or have another healer heal over a HoT, we are wasting spellpower, everytime we overheal with a crit, we're wasting crit. In each of those situations we're also wasting all the other throughput stats. Of these scenarios the 'not pushing buttons fast enough' scenario is the easiest to control. It would be nice to be able to optimize the other two, but they've been around since WoW began and often hinge on making snap predictions about the future and the habits of other healers, and of the person being healed.

There are also cases where you don't want to be chain casting, and in these cases you're 'wasting' haste, but don't really care. It's important to parse those out. It ends up being a somewhat small set of cases where haste waste is significant but a lot of decisions we make for spec, gearing, and suchlike are based around the highest healing required cases, even if they're rare.