Discipline has 15% more Intellect, true, Afflictid. But Intellect is actually really bad when it comes to giving you crit percentages. They have 10% from Renewed Hope, Holy has 10% from Chakra (Serenity). Now Holy has a 100% uptime on the corresponding Holy Word, Holy Priests have an additional 25% crit that Discipline just doesn't have.
And as for the healing on burst recovery, Serendipity actually outweighs two Greaters both in numerical output, as well as "efficient" output (assuming you can manage your overheal), because unlike Train of Thought, Holy doesn't actually have benefits to Greater Heal that make it "ultra-efficient" as a standalone spell. And no, you can't "count on crit to keep a tank alive", but the fact is you NEVER top a tank off (unless he's a Blood Death Knight), because a Shaman's Riptide, Druid's Lifebloom/Rejuv, your own Renew, offtank's Word of Glory, etc etc etc will always come into play. If you aren't trying to keep your tank in the 85% zone, you're really pushing too hard.
You know, people say the same thing about Penance. And yet, how many people actually glyph it? It's not the fact that Serenity alone will keep a tank alive. But the fact that you are critting more often, thus giving you bigger Echoes as well, your Renew is critting more often (and never falling off), you can take advantage of your crits to push that tank to 95% instead of 90, and have a bigger buffer before NEXT BIG HIT INCOMING."Man this 15k heal is totally going to keep tanks alive because it is on a shorter cooldown and gives an increased chance to crit."
The only time where I would recommend against ever trying Holy Tank healing is if you get yourself in a situation where you're healing a Blood Death Knight. Because, at least from my personal experience, everything just lines up perfectly between deathstrikes. Or at least, I found myself in perfect sync with him, but that's anecdotal and nothing more.
Your clock analogy is slightly off. Discipline is no Atomic Clock (which does run dry, and need to be maintained consistently or it's off. There's a reason why it constantly resyncs). Discipline's the Digital Wrist Watch. You can take it with you wherever you go. It's quieter than your Grandfather clock when it goes off, but does its job quite well. It's also easier to read at just a glance (to this day, I have trouble telling time). Holy is loud, you notice when Holy stops working a lot sooner than when Discipline does. You notice when Holy's working harder than when Discipline is, as well. But for the sake of telling you what time it is? Both do their job.