Easy to produce and maintain
Some recent Wysiwyg editors can produce cross browser code (DW 4 & MX), provided one uses the appropriate parameters
By separating presentation from structure, one can change the look and feel of a complete web site in seconds.
Well-structured documents are also easier to maintain :
Logical constraints induced by validating XHTML help maintaining such a logical structure, enhancing the predictability of how your content will display in browsers.