Consider that any link in the chain that connects server to storage could fail. If the applications provided by the server are so important that it makes sense to spend additional money to protect them, then hardware redundancy is an important early step.
Of course, you may choose to have redundant servers, redundant switches and redundant storage. Making everything redundant, and then double-connecting everything together can certainly be an expensive proposition. If all of this stuff were free, then it'd be an easy decision. Everybody would make all computer systems redundant. Of course, life isn't that simple, and it ultimately comes down to a financial decision: Does the cost of downtime for this application warrant spending money on extra hardware, and the software this is required to manage it?
Note that you could get further increases in availability by having three of everything, or even four or five. However, in general, going beyond dual redundancy (except in the case of extremely critical applications) is overkill. Of course, it's still a financial decision, and without knowing a lot more about your environment, I cannot tell you exactly what makes sense there.
Hope this helps.
This was first published in July 2004