Grasscutter is a great card getting Sin Buster over the 30K threshold for his glory skill, but if you have striden Heavyduke, you are not allowed to target him for the skill since he is not a grade 3 vanguard.
By having Ur Buster on the field when you legion, the 4K that Ur Buster gives in combination with Grasscutters 6K will have Sin Buster sitting pretty at 30K unboosted, meaning that even without triggers you can use Laurel without worrying about perfect guards.