Weapon swap > Merciless Resolve creates a long delay which locks you out of casting other abilities.

NightbladeMechanics

If you weapon swap into an immediate Merciless Resolve -- the buff ability, not the Assassin's Will proc -- the buff applies immediately, but the animation does not happen for another second and a half, during which time you cannot cast other abilities or commands.

This locks me out of an entire global cooldown during my rotations, negating the utility of the recent Grim Focus buffs. You cannot weapon swap into Merciless right now.