There are 2 problems with it:
- If the paladin falls under 20% for any reason -- either other damage or damage from BoS -- the external ends. So depending on the damage and/or health of the paladin, this 12 second cooldown might only last 2-3 seconds.
- If the target receives a lot of spike damage, such as a tank taking heavy damage, there is a very real chance it will kill the paladin. There are already plenty of logs of this happening on Beta, and it's not always healable damage... and even when it is, it requires that paladin to spam self-healing instead of doing something more productive such as healing other people.
- Life Cocoon (Mistweaver Monk)
- Pain Suppression (Disc Priest)
- Guardian Spirit (Holy Priest)
- Ironbark (Resto Druid)
The disagreement is whether to even put it on your bars at all, with some insisting that it's never ok to use in any circumstance (anytime there isn't a real risk of dying, it's a waste of an external cd as the damage clearly isn't that high).
Personally, I still see a value in using it, but I still don't believe that it would ever be appropriate to use if you have one of the other options above available or you're specifically planning an external cd rotation with special emphasis on placing the BoS at a specific time when you know the paladin is safe to use it.