Redundant RAID controllers only for reliability?

What kind of setup are you talking about?

- PV220S?
- PV650F?
- PV660F?
- Dell|EMC FC/CX-series?

Also, are you using any kind of failover software? (ATF/QLdirect/Powerpath)

And are you using a single fibre card or dual cards?

A PV660f in direct attach mode, with 14 36 GB drives.

In a PV660, the second RAID controller gives you several advantages. While only ONE controller can own the LUNs at a time, the presence of the second controller gives you a second, failover pathway into the box; it gives you a failover controller in case of the primary's failure, and it gives you mirrored cache for much better performance. Without the second controller, there is no caching, because it isn't protected  by a backup (mirror). And, don't forget to store a copy of your COD (configuration on disk) in case you ever have a problem with the unit.