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