I am in the process of retiring Outlook 2003 clients while simultaneously bringing Outlook 2013 clients online. Until now, I have left Direct Booking in place on mailboxes that are used as conference rooms. All I had to do was enable Direct Booking in Outlook 2010. This is no longer possible with Outlook 2013.
I have seen plenty of posts/articles on how to convert Exchange 2003 style Direct Booking mailboxes to Exchange 2007+ Rooms w/ AutoProcessing set to AutoAccept. What I haven't seen is anything showing the downside of keeping both configurations in place on the mailbox.
I have tested converting a Direct Booking mailbox to a Room resource in Exchange 2007, leaving the Direct Booking attributes enabled, and both Outlook 2003 and Outlook 2013 can successfully book the resource. Outlook 2003 uses the Direct Booking logic, and Outlook 2013 uses the Exchange AutoAccept agent. Both appear perfectly happy.
When I finish retiring Outlook 2003, I will officially decommission all of the Direct Booking attributes. Until then, I'd like to know why this is not supported. Any feedback / horror stories are welcome.