Well I'm actually surprised no one actually calculated this with the real numbers O.o
I guess I'll have to do it

1- Lets assume it's a mage as it's starting stats are more designed for melee then those of a guardian.
2- Lets assume the mage is level 65 with all his points invested into int (75 extra points).
3- Lets assume he has no proficiencies
4- Lets assume he has the gear of an average healer
5- Lets assume monsters don't have defense
6- Lets assume CS don't happen, because I wouldn't know how to calculate them. However since Dexterity isn't changing the % in damage should stay the same so it shouldn't make a noticeable difference (correct me if I'm wrong).
Mage base stats
Strenght- 20
Dexterity- 50
Intelligence- 60
Vitality- 20
Our Mage HEALER stats at level 65
Strenght- 20
Dexterity- 50
Intelligence- 135
Vitality- 20
Heal Mastery- 135 (Equal to his Int for best possible effectiveness)
Our Mage's gear
Magicial Ultimanium Dagger
64-113
+45 Heal Mastery
Magicial Spiderthread Robe
+45 Heal Mastery
Magicial Focused Heal 14
+45 Heal Mastery
Acc.Charm
Magical focused Heal 14
161-193
130 Enhanced Effect
Ok so our average damage attacks are doing 89 damage and our average heals are doing 312
And so our total heal/damage output over 100 actions will be 13360
Ok now lets check out the other option you guys are saying, replacing the HM dagger by one with say 100 enhanced effect as thats how much a 45 heal mastery dagger is worth in enhanced effect about.
Our Mage Hybrid stats at level 65
Strenght- 20
Dexterity- 50
Intelligence- 135
Vitality- 20
Heal Mastery- 90
Our Mage's gear
Magicial Ultimanium Dagger
129-225
100 Enhanced effect
Magicial Spiderthread Robe
+45 Heal Mastery
Magicial Focused Heal 14
+45 Heal Mastery
Acc.Charm
Magical focused Heal 14
161-193
130 Enhanced Effect
Ok so our average damage attacks are doing 177 damage and our average heals are doing 252
And so our total heal/damage output over 100 actions will be 19200
So your being about 70% more effective :P However the amount of total heals you can give drops by a lot.
At level 65 your mana pool will be 6780. So you can give 67 heals so thats :
Healer - 20904
Hybrid - 16884
Your losing 20% of your healing ability for 70% more overall effectiveness.
-------------------------------
Personally though, Guardian tank + mana syphon + healer will be the best possible healer if built correctly as it basically has infinite mana pool as well as serving 2 purposes in a cata, tank and healer. I'll admit though they won't work on a budget.
Hope this helps
Thanks for you long explanation.
imo..the best healer is a healer that can last the entire game..a healer is there to keep the others alive...if hes gonna be doing the killing aswell he might aswell solo
I also think so.