Replacing apps on desktop/laptops is ok. But personal devices (some of them also use CalDav and CardDav) cannot really replace installed/embedded Apps.
Also: import routines for addresses, mail or calendar are often file based, but sometimes most of the data lays on apps that can be connected to other apps (Outlook, Thunderbird, other PIMS/mail clients) which can be really nice to connect with protocols to mailserver (therefore, with the Groupware).
Mail server lays on system, but calendar and address book are on the groupware.
ActiveSync is good. But it’s not for everyone…