Now that we have this cool Validation Application Block we want to use it with all our business objects. This should immediately send up a red flag that there will probably be some common functionality among all these business objects in how they use the Validation Application Block. Hence some type of layer supertype ( business base class ) will probably be necessary to contain common functionality to remove code redundancy.
Source: David Hayden [MVP C#] : Validation Application Block - Integrating It Into Your Business Layer
David Hayden presents another great article, this time on the newest addition to the MS EntLib: Validation Application Block.
I go back and forth on the EntLib as a whole. Parts are great, other parts are overkill. I love the Data Access Block and use it in tandem with my other data access frameworks (ActiveRecord, NHibernate, etc). The UI block though, WTF!
David's article is a good read. Go forth and read.