Body Count does not affect weapons generated by Warframe Abilities such as Exalted Blade, Hysteria and Primal Fury. This is because Ability-generated melee weapons use their own melee combo counter separate from the player's melee weapon, and its duration is capped at 3 seconds. The combo counter of the player's melee weapon will be preserved provided the player deactivates their ability before the combo counter refreshes.
This mod's increased duration can be paired with the increased combo counter hits provided by Excalibur's Surging Dashaugment, which in combination allows for larger combo hits, and consequently greater bonus damage to be dealt for longer periods of time.
Ash's Blade Storm benefits from a longer combo counter time to maintain greater damage numbers from the combo multiplier for much longer. This can be taken even further by using the Rising Storm Augment.