The biggest thing that most people I talk to about Free Busy don't understand is that the generation of the Free Busy data is the responsibility of the client not the server in the vast majority of cases.
To help people understand this it is usually best to just walk thru a publishing scenario.1.
The only time where Free/Busy replicas really makes sense is in the case where you will have people regularly needing Free/Busy information from others that are in a different physical location and are separated by a very slow link.
In this case they would not be able to make a consistent connection to the public folder server in the other site to get the needed Free/Busy information.
Any time you add a replica of the Free/Busy folder to another server you will introduce replication lag into the mix.
This Public folder replication lag will ensure that the replica of the folder does not have exactly the same information as other replicas and thus your Free/Busy information will not always be accurate.
The process that is responsible for this is called Mad FB or MSExchange FBPublish.
Mad FB then publishes the users full Free/Busy message to the Free/Busy folder overwriting the existing message.If I make a change to my calendar in OWA it still updates my Free Busy but my Outlook client was not running, how does that happen.In that case since we do not have an outlook client running it does become the responsibility of the server to publish the clients Free Busy information.If you find that the message is completely not there then I would recommend first refreshing the client Free/Busy information.The easy way to do this is to log into the clients mailbox with outlook using the /cleanfreebusy switch when launching outlook (this is easily done from the run window).