The only time its useful is in *extreme* edge cases that almost have to be designed to make it effective and, even then, the improvement is so minute you'd likely never notice. It's only going to be legitimately useful for 2 levels, from 28 to 30, when you've got Enhanced Fracture (so the duration is 30 seconds and the potency per GCD it provides is 300) but before you've got Butcher's Block (which turns the HS>SS combo from less efficient than Frac to way *more* efficient than Frac).
The only possible way I could see for Frac to be useful would be for some kind of proc trait on the DoT damage be applied, likely something that restores TP or allows recharges Mercy Stroke and allows you to use it without the target being below 20%. They could change either Enhanced Fracture or Enhanced Mercy Stroke and, interestingly enough, it would actually make the both of them useful if the proc chance is high enough (20% proc chance at 6 ticks would boost Fracture's effective potency per GCD to 460, which is enough to make it cost effective).
I actually kind of like that idea. Change Enhanced Fracture to the 20% chance per tick to reset and allow Mercy Stroke and Enhanced Mercy Stroke to provide 5-10% of max hp on a hit regardless and you've just made the both of them actually have a place in the grand scheme of WAR.