The controller is a translator that sits between the hard disks and the computer. There are two reasons to use multiple controllers for a single RAID array: 1) To alleviate a performance bottleneck, or 2) to provide for redundancy in the event of a failed controller. Reason #1 probably doesn't apply to you, unless you're designing an array with more than six drives. So, if you decide that you need every component within the server to be redundant, get the redundant controller too.
I feel like I should warn you -- most people buy far more hardware than they actually need for their site. Your software is about 100 times more likely to fail than your hardware, even if you don't use redundant components. So, be conservative when you consider the costs of adding redundant hardware.
This was first published in March 2002