Firstly - WebDAV is going away. See e.g. this blog entry by Terry Myerson, General Manager for the Exchange Server division at Microsoft.

Secondly and simply, WebDAV is not an option if you are one of those who are still on the Exchange 5.5 platform. A lot of organisation are still on Exchange 5.5 . Using Exchange 2000/2003 will give you more of the native feature set from Outlook, but a WebDAV based client application will allways play catch-up to a MAPI or Brutus based one.

WebDAV does require you to enable IIS as well. Enabling IIS is something that a lot of IT departments would rather prefer not to do. Admittedly, Brutus would require the Brutus Server to be installed on some Windows server too, but Brutus is Open Source and subject to constant third party scrutiny for software bugs and security problems.

Another problem with WebDAV is that the protocol implementation on the Microsoft Exchange Server side of things is far from stable API-wise. A lot of features are not supported on earlier Exchange versions and supporting all Exchange versions from 5.5 onwards is simply not feasible for a real-world WebDAV client application.

An argument against supporting all Exchange versions from 5.5 onwards, that I have heard, is that Exchange 5.5 is dead technology and that I might as well focus on Exchange 2000 onwards. This is far from being true. Exchange 5.5 are alive and well in many server rooms. Many organizations would really prefer to keep their Exchange 5.5 despite Microsoft discontinuing offical support.

The "good" thing about Exchange 5.5 is that it is known inside out by most Exchange admins. An acceptable option is therefore to isolate the old Exchange server behind trusted firewalls and just keep it running for as long as possible. Brutus enables an organization to use alternative desktop clients that support a multitude of backends, thus migrating away from Exchange in small manageable steps instead of one giant "upgrade to Exchange X.Y" transformation.