Disclaimer: This does not include veteran content where bosses do ramp up more aggressively when in their execution phase, the companions would probably be dead most of the time anyway.

The way I look at them, Slayer's Blade for Mirri and Sever from two-handed weapons, are pretty awful even with their short 8 seconds cooldown. Short fights with weak enemies is so insignificant the damage doesn't matter, they're going to die quickly regardless what you have slotted. During long fights your companion basically have an ability slot that is inactive for the majority of the fight. You could fill that ability slot with another normal damaging ability they would use in their rotation throughout the entire fight (not just the last 25% health) and accumulate as much damage or more than what the execution would have contributed overall.

For Mirri I would argue that you would better be off using Parallel over Slayer's Blade to get increased Ultimate build up to use Impeccable Shot more frequently which does the exact same damage as Slayer's Blade with a 3 second debuff for 20% more damage which is also available the entire fight not just the last 25% health. Companions essentially lose an ability slot and have a shorter rotation with roughly the same amount of downtime where they're auto attacking. I would prefer more damage from a normal ability that continues to be used throughout the fight not only at the end, that gets you closer to the end faster than prolonging the fight with an ability that is inactive most of the time.