Hello,
This topic has been mentioned before but I can't help but to mention it again, since none of the answers I've seen are satisfactory..
If you look at the BLL classes and the DAL classes you're notice a lot of repetition (for instance Articls and ArticleDetails share many properties).
Now, as I sit down and type my code, something inside me tells me I'm doing something wrong.. I keep copying and pasting. All this for the sake of separating layers ?
Why don't we have some class called "ArticleInfo" which becomes a member object in both ArticleDetails and Articls and contains the common structures ?

Separation of tiers is done to allow for future re-implementation without affecting other tiers. Sometimes a BLL class adds no new functionality, but maybe someday it will. If the UI called the DAL directly, then it would be harder to add new business logic in the future.

You can check out my download slides on ASP.NET architecture if you'd like. You'll find some more info about nTier there:
http://www.EricEngler.com/Presentations.aspx

Eric