I was playing the mission 'The Crucible' and they had a mechanic 'Click to Spawn Locusts'. This reminded me of 'use artifact' from the last mission from Wings of Liberty.
There were achievements associated with the Wings of Liberty mechanic so I tried to hold out as long as possible without using it but had to in the end. Are there any reason why you should not use it? Like a Feat of Strength or something?
No, there is no "real" reason to not use the Locusts. The only achievement associated with Locusts for Crucible is:
Epic Meal Time: Kill 75 enemy units with Primal Spawn Locusts.
It could be a user preference though, I know at times when I replay the mission I like to make it 'harder' by NOT doing what they want you to do. For example, when they introduced the Roach I beat the mission solely with Zerglings. Sometimes just having a mission on Brutal is not hard enough.
The achievements for "The Crucible" are:
The Crucible
Complete “The Crucible” mission in the Heart of the Swarm campaign.
Epic Meal Time
Kill 75 enemy units with Primal Spawn Locusts in “The Crucible” mission.
Can’t Touch This Chrysalis
Complete “The Crucible” mission without Kerrigan’s Chrysalis taking damage on Normal difficulty.
Short Life Expectancy (Mastery)
Kill the Tyrannozor in under 1 minute after it appears in “The Crucible” on Hard difficulty.
the Locusts are necessary just for the "Epic Meal Time" achievements, otherwise the mission can be completed without using them. But there is no particular reason to not use them
Blizzard added an achievement for doing this on the 10th anniversary of Starcraft 2:
Entomophobia
Complete "The Crucible" mission without spawning Primordial Spawn Locusts on Normal difficulty.
