TFA quotes the e-mail:
In the rare case you do need Outlook, like adding a delegate for your calendar, you can still fire up Outlook for 30 seconds.
Is that feature so important that it has to mentioned? It certainly doesn't seem like it should be months of work to add it to the in-house webmail client if Outlook can do it! I'd certainly press for having the feature added to the in-house solution *before* sending out an e-mail lambasting employees for not using it!
What I find really funny is that Zimbra belonged to Yahoo, and Zimbra definitely can add a delegate to a calendar.