So, I'm not terribly keen on giving Clemency an insta-heal either... but this doesn't follow.
If we're talking about pure healing capability, than the fact that Pld's even have clemency steps on the healers toes (since they can self-heal or heal other teammates), so, by your logic, Pld's shouldn't even have that move... and War's should not have inner beast or equilibrium (which effectively allow them to self heal sufficiently without healers at all in many circumstances). This is especially considering that Clemency can save a Pld, even without insta-cast.
There's nothing to stop a Pld from beginning the cast during the cast bar of a tank buster and timing it in such a way that the heal goes off immediately after the hit (I do this all the time). It's true that sometimes the cast gets interrupted, but you'd be amazed how many tank busts don't actually interrupt cast bars. It's actually the auto attack. So, if the timing is right, Pld's can already save themselves from high dmging tank busters. This would just provide for a larger margin of error.
Now, if we're talking about "saving the day" (which really sounds like an ego thing, but w/e), than it still doesn't follow. Again, Pld's can already save the day (for themselves or one other unlucky target) if they know how to time their clemency correctly... or even a well used stoneskin.
It used to be super popular for Pld's to apply SS before big cleaves in order to buff themselves against tank busters. Why was this important? Because Tank busters are traditionally followed or preceded by big AoE dmging moves. The Pld's ability to be self sufficient for the immediate seconds before or after the tank bust allows the healer to "save the day" for the rest of the raid, especially because Dps are known to panic and stand in stupid. The healer's job doesn't just end at the tank. If I self heal a tank bust, the healer shouldn't be moping around, lamenting that they didn't get to shine for a minute, they should be looking after the rest of the group and applying buffs.
The Warrior comparison doesn't make sense either. I have seen and been in fights where the Warrior's outgoing Dmg was the only reason the group made the dps check. How do I know? Because two of the Dps were dead and we still made it. The Warrior was working a PvP burst dmg rotation (you can look it up in the PvP forums if you want the exact rotation. It's pretty impressive, and has an obscenely high output), and that's more than enough to make a difference.
So, the question is, why is the high dmg output of a Warrior, something which has been proven to be an effective means of meeting DPS checks (which are supposed to be the responsibility of the dps... kinda in the name) and is now considered the Meta for clearing high end raids (at least until gear levels get higher) somehow not considered stepping on the Dps's toes, but a Pld self healing through a tank buster (something that a Warrior can already do with the right cooldowns), is somehow considered stepping on the toes of the healer?