The removal of the weapon damage stacks from Merciless Resolve ( and Relentless Focus morph ) was made to combat the skill line's popularity and strength in subclassing, both for PvE and PvP. But a lot has changed since then.

1st impactful change - we have class masteries, making subclassing no longer the one and only go-to to stay competitive.
2nd important change - the DK rework.
Molten whip has the exact same tooltip as merciless when fully buffed, except:
- It's easier to stack. Doesn't even require me getting into any sort of danger. I can just heal while I run;
- It has no travel time due to not being a projectile, so it's more reliable to land since it takes less time to actually hit the target;
- It actually gets significant buffs to make the damage actually higher ( 5% ardent flame passives, 5% encratis, vamp passives etc. );
- It does NOT get completely eaten by skills like ball of lightning;
- it doesn't deal trash damage against DK wings, which effectively reduce the damage of Merciless in half. Not only is 50% or more of the PvP population running DK, but pretty much every DK runs the wings skill anyway;
- it doesn't get mitigated by projectile sets like beckoning steel.

It's definitely not the first time when you nerf something but neglect reversing the nerf when it's no longer relevant, leading to tons of subpar skills / sets etc.

And if the damage increase is so significant in PvE that it's gonna destroy all balance, you can always tag a battle spirit being active requirement on it.