i use 2 arrangers for 2 simple reasons:
1- Versatility, Different styles and sounds
2- Backup
can't believe more people don't think of a backup plan.
arranger as an electronic could suddendly die on them, or decide not to power, or have a memory zap, etc...
i couln't imagine being at a wedding gig without backup...
so i use a Roland G-70 and a Ketron X3, so i am totally covered in case the worst happens