If I may, I would like to offer my experience with Praise and Worship, as well as being organist/keyboard/music director of many churchs over the past twenty years.
Believe me, I am not a know it all, but have run accross many situations.

If you are playing praise and worship, and you always have drums,guitar, or even a good gospel piano player, you probably don't need an arranger. However, I have been with groups and turned on a simple 8 beat, for example, and they loved it. Like Scott said, it gives you flexibility. I have played at many churches that didn't want drums, but wanted my accompaniment instead. If you are playing service music, that is a different story. There are plenty of praise and worship, or Southern Gospel groups that plain get down.

The bottom line is, if you will always have full backup, then you are wasting your money. If you think you may hit other situations, the arranger is always there if you need it.

Thanks for letting me share.

Bernie