You rolled a holy pally, too? =)

^^ This post exactly.

In raid healing only a few bosses actually allow the use of Greater Heal.
I'm sure if you go and run say Naxx with an Ulduar geared tank you can do it, but with a tank that's in the gear the should be, when do you have the time?

If you wait to throw the heal so that 80% of it is not overheal, your target is likely to be 60% or lower where a little burst damage from the boss will either drop them or make you waste the mana you just saved burning flash heals to be sure they're up.
Yes, bosses still burst. I wouldn't 'exactly' call it like the old crushing blows but since tanking is primarily dodge/parry based, a little fail of RNG on your tank gets them hit consecutively and often with abilities their wonderful armor doesn't mitigate.

I end up using flash heal for most of my raiding and I absolutely hate it.
Healing is supposed to be reactionary. The tank takes damage, you heal it, rinse repeat.
With bosses dealing such huge amounts of damage most of the time you're just throwing heals repeatedly just to keep the tank 'near' capped off so in case something happens (like burst or boss abilities that incapacitates) they don't drop before you can get back to them.

Healing in raids, in my experience, isn't the art of healing and careful ability use. It's spamming whatever is fastest, shielding if it isn't enough, and judging if you might get something out of a CoH. Besides that it's boss gimic: avoid this, heal through this, don't heal during this, only heal during this 3s window. Continue with praying to RNG for your tank and hoping your DPS is up to par.