Lets take into consideration on how many boss fights do the treants survive more than 10 seconds or if the boss is ''available'' for dps over 30 seconds in a row, without any aoe around him.
Boss - "available for 30sec just after heroism cast yes/no" - Aoe?
Anub'rekhan - Yes --- Yes impale might kill them, in reality this is a pretty low risk though & they usually survive.
Faerlina - Yes --- There's aoe, but if they die its due to a poor tank/lack of healing & not the encounters fault.
Maexxna - Yes --- No aoe.
Razuvious - Yes --- Distrupting shout wont kill them.
Gothik - Hard to get a full 30sec duration. --- No aoe (looking at the only meaningful casting time).
4h - Yes --- Very strategy dependant, in some cases they will survive full duration & in others they get nuked by 3 boss casts (allthough quite an amazing tool to stabilize a pull / safe ranged tank from slacking healing).
Noth - Yes --- No aoe.
Heigan - Yes --- Tank dependant, but treants will die & fairly fast.
Loatheb - Yes --- Nothing deadly.
Patchwerk - Yes --- No aoe.
Grobbulus - Yes --- Nothing deadly.
Gluth - Yes --- No aoe.
Thaddius - Yes --- Phase 1 nothing / Phase 2 they will die.
Sapphiron - Yes --- They will die.
Kel'thuzad - Yes --- They usually die, but lasts longer than some of the other certain death fights.
Sartharion - Yes --- For 3D perspective they are likely to die in a fissure, but on the other hand brambles helps with adding burst for where it was needed when we had level 70 gear.
Malygos - Yes --- They usually survive.
17 bosses, possible on atleast 16 of those to dps for 30seconds after you popped heroism.
In 10 of the 17 fights chances are very high that they survive for their full duration.
In some cases you can be sure that they wont (Sapphiron, Thaddius p2, 4h some strategies, Heigan) & in others theres a pretty high risk (Sartharion, Kel'thuzad).
So it becomes a matter of preferance, I find it very hard to justify speccing Owl'Kin frenzy before Brambles, but both talents are in the weaker dps department & likely to be dropped if you want more utility or need mana.