Why would it to be good for organizations to make use of blogs as a substitute for weekly reports and updates? Imagine how much of a difference it would make:
1. Search would be possible (No need to look through email folders and worry about misplaced emails or lost emails)
2. Tags would make it easier for people to locate updates related to a particular activity
3. When the employee or her manager leaves, the new person who takes over will have a wonderful record of the project's history and milestones/challenges etc!
4. Would be easy to invite third parties to take a look at project progress when the need arises