There are many different reasons why you would or wouldn't want to use a server-class motherboard in a workstation, and it all depends on the application of said workstation.
For example, the Intel S5000 series motherboards are great for high-performance number crunching, and utilize the Xeon processors. This is great for video edition, CAD work, etc.
On the other hand, an S3000 series motherboard allows for the Core series processors, but still has the server/workstation class features and driver support, but is useful for less intense projects, such as file-serving, etc.
When you move to server class products, you receive different software and OS compatibility and support. If you are looking for a server or workstation, perhaps a motherboard bundle might be the best way to go. I am sure there are several places across the internet that provide them. The company that I work for,
www.jncs.com, provides server class bundles if you are interested.
Hopefully I have been helpful.