As soon as you start considering Rage of Halone, you're basically in a situation of a "sliding scale" between Enmity, DPS and Mitigation, and there's a number of variations. It's up to you to decide which you want to use, but here's a breakdown.
Let's do all of this theorycraft with the assumption that the paladin has a 2.4 second global cooldown. This is a lot of skillspeed, but with averages from Selene's buff and such, it's certainly not too bizarre to consider. The reason we want to test with a 2.4 second global cooldown is that this lets us assume that Goring Blade (which is a 24 second duration DoT) has 100% uptime of all ticks whether you're using a 9 or 10 global cooldown rotation. If you have higher than 2.4 sec gcd then a 10 gcd rotation will always lose a tiny amount of DPS based on the "gap" in the reapplication of the DoT, but it's minimal and very hard to calculate - a 3 second gap would lose a whole 40 potency tick, a 1.5 second gap would lose on average 20 potency etc. So let's just assume 2.4 sec GCD and 100% Goring Blade uptime.
We then have to calculate the Potency and Threat of our combo options. Reminder that Savage Blade and Rage of Halone are x3 and x5 threat modifiers, so they do 200 and 260 potency of damage but 600 and 1300 potency of threat.
Goring Blade combo Damage = 150 + 230 + 220 = 600 Potency Damage over 3 GCDs (plus maintains its DoT)
Rage of Halone combo Damage = 150 + 200 + 260 = 610 Potency damage over 3 GCDs
Rage of Halone combo Threat = 150 + 600 + 1300 = 2050 Threat over 3 GCDs
Royal Authority combo Damage = 150 + 200 + 340 = 690 Potency damage over 3 GCDs
Royal Authority combo Threat = 150 + 600 + 340 = 1090 potency threat over 3 GCDs
Fracture = 220 potency over 1 gcd.
Note that we never ever have to consider Shield Swipe, Mercy Stroke, Circle of Scorn or Spirits Within in working out a rotation - these are all off-gcd moves and will always be popped on cooldown. Changing the rotation of GCDs you use doesnt affect how you use the off-gcds.
With a 320 potency over 24 seconds, the Goring Blade combo will ALWAYS be used in any remotely optimal rotation unless you really are ONLY concerned about threat - in this case, spamming Rage of Halone combo constantly is by far the highest threat option, but very low damage. Therefore, our rotation choice are limited to: Goring / Royal / Royal, Goring / Halone / Royal, Goring / Halone / Halone (as 3 different 9-gcd rotations), and those 3 rotations with a Fracture inserted to make them 10 GCD rotations. Any other option will make Goring Blades DOT fall off for too long.
So let's look at the Average Potency per GCD for Damage and Threat for each of the 3 rotations with and without Fracture:
Goring / Royal / Royal = 220 Damage and 309 Threat
Goring / Royal / Royal / Fracture = 220 Damage and 300 Threat
Goring / Halone / Royal = 211 Damage and 416 Threat
Goring / Halone / Royal / Fracture = 212 Damage and 396 Threat
Goring / Halone / Halone = 202 Damage and 522 Threat
Goring / Halone / Halone / Fracture = 204 Damage and 492 Threat
You can see that in any 9 GCD rotation which involves a Rage of Halone, you will SLIGHTLY increase the damage per GCD by inserting a fracture, but you'll greatly REDUCE the threat per GCD by doing so. If you're not doing the maximum damage rotation of Goring/Royal/Royal then you're needing either extra threat or keeping up the Halone debuff, and you can see from the values that it's going to be much much more efficient to just slip in some Goring/Halone/Royal combos in between your Goring/Royal/Royal ones and ignore fracture entirely, as you'll gain much more threat per GCD by doing this, and this should enable you to pull ahead in threat comfortably enough to then swap back to the optimal dps rotation.
There's one other reason too - if you're keeping up your Rage of Halone debuff... remember that this debuff lasts ONLY 20 SECONDS. If you have a 10 GCD rotation (ie, one that includes Fracture), and do only a single Rage of Halone in 10 GCDs, you'll have 20 seconds uptime and 4 seconds without Rage. If you use a 9 GCD rotation of Royal/Halone/Goring then this is 21.6 seconds and thus you only lose 0.6 seconds without the Halone Debuff. You actually need a 2.33 second GCD in order to have 100% Rage of Halone uptime with a 9 GCD rotation, but generally dropping Halone for 0.6 seconds is fine.
I guess you can probably say that Fracture has a use if you absolutely MUST have 100% Halone uptime but care more about damage than threat - in this case, doing a Goring / Halone / Fracture / Halone rotation will keep the Rage of Halone debuff up 100% with no gaps and do sliiiightly more damage than just Goring / Halone / Halone. But given the Halone debuff is 8-9% damage reduction only, I can't see this being viable. There's really no encounter in the game at 60 where I can see this rotation being useful (though of course, this is technically the optimal DPS rotation for all pre-60 content as this does the highest damage per GCD without having Royal Authority). The Rage of Halone debuff just isnt that useful - it's good in certain situations where you want to reduce specific attack damage, like applying it before a Blade dance/Heel on Thordan or when you've got a lot of A3 vuln stacks, but 100% uptime of the debuff isnt ever a requirement enough to justify dropping Royal Authorities from your rotation.
TLDR Rotations:
Goring / Halone / Royal = optimal mix of Damage, Threat and Mitigation.
Goring / Royal / Royal = maximum DPS.
Goring / Halone / Halone is for when all you care about is threat/mitigation.
Insert a Fracture as a 10th GCD if you're under level 60.
Also, as an aside, you mention Mercy Stroke - I assume you arent considering swapping between Mercy Stroke and Fracture on your crossclasses? Mercy Stroke, Foresight, Bloodbath and Stoneskin are ALWAYS your must-haves. If ever you put Fracture in for some reason, you'll have it in addition to those, probably swapping out Protect as Cure has no use past about level 20 (as this thread shows).
