"No one single-heals Sindy"

WoL would disagree with you. (there are logs of a 1-tank, 1-healer, 8 caster DPS zerg strategy on 10man, and these are the top dps'ers for that encounter).

More to the point, if you are doing it with 3 healers, when you have 1 with unchained, and one that is in a tomb, you are now effectively solo healing. This is one of the few encounters where triage is really what healers are doing in wrath; I've sat at or below 50% health on that fight more than I care to remember.

"A raid will only progress as fast as the slowest player."

One below average healer on sindragosa hardmode basically means you can't do it, because when that person becomes the solo healer, you're raid is going to die.

It's true that you can trust your other healers, but they may have also blown most of their CDs, or have a hard time healing a lot while moving just like you. A druid could probably save the tank and run fairly well (Rej+Swiftmend+Lifebloom stack - 2 GCDs would probably get the tank out of trouble, and frankly a druid probably would already have a HoT on the tank to swiftmend with), but other healers would probably have trouble, especially if they don't have their instant CD buttons because they've already used them (Pain Suppression, Guardian Spirit, etc).

Consider this a complete 'worst case scenario', and take the article form that angle, rather than a 'this person obviously feels they must heal everything' angle, imo.

When I read this I was thinking, "Yah, that's how I am sometimes!" but also feel that it's a push to be more competitive/ effective as a healer. And what if both healers were expecting the other healer to pick up the low HP person.. then you get the 'OMGWHYDIDN'TIGETAHEAL!?'. As a tank I would appreciate more seeing BOTH healers blowing CDs on me to keep me alive. That would encourage my trust in them more to know that all my healers have good reaction time.
I tend to find the 'trying to do too much' less of a problem and more of a positive trait. I mean.. why be bored as a healer if the DPS always gets to go 'balls to the walls' and the tanks are always swinging away for threat generation. Hopefully you'd never see a tank ignoring adds swinging at the clothies just because 'ohhh I assumed the other tank would pick them up so i didn't do anything'. And by then all the clothies are dead. And mad. >:(
So if all the others get to have excitement and be able to push themselves to the limit, why shouldn't the healers ? :D

Also remember you don't always have a pally in the raid, or a disc priest or a druid. I've seen 25 mans without a priest or without a druid. And especially in ten mans, The shaman MAY very well have been brought to Tank Heal. No class alone can be said to only be able to heal one specific thing in one way. We all get a whole plethora of abilities that allow us to be good healers in many different situations; you just have to understand how to utilize and apply those abilities.