This is wrong on so many levels. Let me give you SIX reasons why Cocoon isn't viable as only healing ability:
1) Based on Temporary HP. If you get hit for 150+ HP when cocoon is active, you lose it and gotta wait a 12 second cooldown.
2) 12 second cooldown themselves. Heal scrolls have a 6 seconds cooldown.
3) It's a Heal over time effect. When I'm at 60 HP, a Cocoon is followed right after by a scroll. Else, you are gonna die anyway.
4) Scrolls can be used on other character while not having a huge penalty on yourself. What I mean with this: use Cocoon on a 1HP friend in your party -> a mob hits you -> you are without a healing option.
5) You need to slot Devotion to have the best use of it. Devotion comes only in Red slot, so it's hard to slot. While this is a minor problem compared to the other 4 reasons, it's still one of them.
6) It's an Epic destiny ability. You have to: buy them, farm them and twist it.