You're crazy. I'm a pretty good sorc healer as most of GAF Empire (and the rest of the Imps) can attest...but to have a group full of sorcs healing hard mode and nightmare, while doable, is asinine. The kind of stacking damage reduction a mercenary healer brings to the table can't be replicated by a couple sorcs. You'd be majorly gimping yourself.
Resurgence, the main combo skill of a sorc healer, allows you to stack 10% armor on a tank. Supercharged Gas, which is roughly the equivalent mercenary skill, allows you to prevent 10% of damage. Which would you rather have on the tank? Me, I'd rather prevent straight damage.
When we do our 16m hard modes, we run 2 mercs and 2 sorcs (also because we seem to have no ops healers) and it works great. Sorcs are better raid heals, mercs are better tank heals. Sorcs pop shields on tanks to help out, mercs use their Kolto Missile and such to help raid heal in spots. It's nice synergy.
All that aside, yes the common perception is that we are by far and away the best. My experience has been that anyone who knows what they are doing in another healing class is just as capable.
You're both right and wrong regarding healers.
There is exactly one fight in the game that we have found where a bounty hunter is a more useful tank healer. That's it, for every other encounter a sorc is a better choice.
When dealing with the heal armor/resist bonuses. You are right, technically a bounty hunter can give a tank a few seconds of better damage mitigation.
As a sorc you have an instant heal that applies a 9sec armor bonus, this heal is on a 6 sec cooldown. As a bounty hunter we have a cast time heal that also has the 9sec armor bonus , this heal is on a 9sec cooldown.(7.5sec with 4piece set bonus). The bounty hunter advantage comes with the use of Supercharged Gas. By burning all 30 charges, our Area heal gains an added benefit. The heal Kolto Bomb ( 6sec cooldown) will provide 10% damage resist if cast during with supercharged gas running. The problem is that we cannot have supercharged gas running all the time. It takes time to get to 30 charges, either we use rapid shots which heals for very little and gives 3 charges, or use our rapid scan which gives 6 charges. Rapid scan is the faster way, but it is also very costly (25 heat), Only by weaving in a mix of both and emergency scan (builds 0 charge) can we get our 30 charges without overheating to do it. This means we can at best keep the 10% resist buff up on a tank at best 50% of the time and usually far less than that. This is because supercharged gas is also one of our primary means of keeping our heat low ( use vents 16 heat) and we usually most save its use to keep our heat in the 0-40 range.
Sorcs are far and away the best healers overall. They have a massive power pool, have easy ways to recharge their power, and have an area heal that is leagues better than other area heals.
I would kill for the power management of a sorc, the massive pool of 650 force with talents and set bonus alone makes our piddly 100 heat seem like a joke. Not to mention a lifetap to restore force power, that talented does not drain like if you crit before hand and with end game gear you should be critting 40+% of the time. This gives sorcs the ability to burst heal, heal nonstop for long periods of time, and still keep going nearly forever. A sorc's single target heal costs 30, not counting regen you can cast it TWENTY ONE TIMES before running out of power. Contrast that to a BH which not counting regen can cast their single target heal FOUR times before being completely out of power.
Contrast that to the powermanagement of my BH. 100 heat of which I can only use the 0-40 range or I suffer huge regen penalties. My normal single target heal costs 25 heat. Casting my only heal that isn't on a cooldown timer TWICE is enough to put me into penalty regen. I constantly have to make decisions a sorc doesnt have to worry about. Do I try to save a dps by healing and push my heat up to high, or do I not heal and hope they can save themselves with a cooldown for a few seconds so I can heal them when I have heat to spend. The worst part is knowing that even a slight mistake early on will cause you to slide down a very slippery slop of penalty regen. 0-40, 5 regen, 41-60 3 regen, 60-80 2 regen, 81-100 1 regen. Once you make your first mistake you only dig yourself deeper into the hole. You have heat reducers, vent heat which can vent 50 heat(66 talented) and is on a 2min cooldown timer, and the previously mentioned supercharged gas which vents 16 heat. And like I said before Supercharged gas has other purposes which forces you to make tough decisions on its use.
On to the Area heal, a sorc heal can hit up to 8 targets, BH area 3, Op area 4. But the real kicker is the effectiveness of the heal. Tests were run on the amount healed between equivalently gear healers using their area healing. The results were that only 3 targets the Sorc heal would heal double the amount of healing per second as the bounty hunter heal. The sorc heal was also 3 times better than the Op area heal on 4 targets So eventhe same number of targets the sorc heal is much much better. Adding in the fact that the sorc heal can and often will heal 8 targets in a raid compared to the BH 3 you can see just how wide a gap there is between the heals.