Yes.

To be more specific... You simply can not use WebDAV if you plan on having a decent feature set and at the same time support Exchange 5.5. Exchange 5.5 has something like 40-60% of the entire Exchange market share. Ignoring that kind of a market share is not something you should do lightly.

Another factor to take into account is that WebDAV/OWA does not offer the same feature set as a Brutus based framework (see here ). Some features are not supported at all (e.g. Journal items) and most features have varying support dependent on the specific Exchange version being deployed. Brutus, being based upon MAPI, does transparently support all Exchange versions equally.

A great feature comparison chart comes from Microsoft. They are comparing the feature set of Outlook 2003 versus Outlook 2003 Web Access. You can see that there are many features in Exchange 2003 that are only available using a MAPI based client. I base this claim on the simply assuption that Microsoft has attempted to exploit the available feature set in OWA to its fullest. You can read the feature comparison here.

Yet another factor is performance. WebDAV/OWA encurs a heavy performance penalty on the Exchange server and the network. This is especially true for Exchange 5.5 (as the link states) but Exchange 2000/2003 has performance problems too, although official documentation from Microsoft is hard too find.

One gem can be get from the source though - A whitepaper from Microsoft compares the performance of Outlook 2003 and OWA 2003. The numbers are not directly comparable due to the fact that Outlook on MAPI and OWA simply doesn't do the same things when connecting to an Exchange server. Outlook 2003 on MAPI does more.

We could look at the simplest test case where Outlook 2003 and OWA are simply started and that there are no new messages. This is the "LOGON5" and the "LOGON2" scenarious respectively. The results shows that OWA generates approximately twice as much network traffic compared to Outlook 2003. OWA 2003 is the higest performing OWA yet, so the performance penalty is only going to be worse if you look at Exchange 2000 and below.