1000 is actually a small inner beast heal. At my level (48) I can hit 1000 inner beast heals on a non-crit by popping berserk. My biggest so far was 1500 on a crit with berserk up (and of course Maim). I expect the norm to be higher at 50 with better gear.
Still, if the threshold is 12,500 that makes it look pretty damn good for using inner beast. for the healing to be worth it you would basically have to get back to 5 stacks before you receive 12,500 healing. But even then I think your numbers are off. Your healing bonus is actually active and changes over time as you perform your combos.
I think ideal (in terms of enmity) will be (ignoring cooldowns):
Inner Beast -> Storms Eye Combo (2 stacks) -> Butcher's Block Combo (4 stacks) -> Skull Sunder (5 stacks) -> Inner Beast -> Butchers Block (1 stack) -> Storm's Eye Combo (3 stacks) -> Butcher's Block Combo (5 stacks) -> Inner Beast.
Realistically you wound need a Fracture in there somewhere, or at least it might be useful. It could maybe be more ideal for dps or whatever but let's just use this as an example.
Anyhow the point of this illustration is that if 15% bonus heals is the "baseline" (making 115% base), then for each stack of Defiance you're missing it's like having a 2.6% healing reduction (13% at all missing (15/115)). So for the first 2.5 seconds (assuming no skill speed) you have 13% reduction. Then 10.4% for the next 2.5, then 7.8, for the next 2.5, then 5.2, then 2.6 then you're back to 115% heals.
The point is it's going to be constantly fluctuating.
I think the bottom line is that in a 4 man it will always be worth it to use Inner Beast as long as you can use the heals. In an 8 man it will depend on how the healing goes. 2 people spamming single target tank heals would mean that it's situational (possibly useful for emergencies and/or big spikes). In the 24 man raid content it will again be based entirely on encounter design. The more healers you have spamming cure 2 the less useful it will be to use inner beast. But if it's a situation where you only are "allotted" one healer, then it would be useful/usable.
**note** it will be interesting how this goes once it's more common to be in the highest endgame gear. I'm not sure if Defiance will scale as well with high ilvl endgame gear as Shield Oath does.