Considering that this is already available thru the webmail (owa) would be great if it could be added

https://technet.microsoft.com/en-us/library/bb936726(v=exchg.80).aspx

https://Outlook Web Access URL/owa/SMTP address of the other mailbox/?cmd=contents&module=calendar

This is my showstopper to fully switch to hiri, as we have a lot of "other peoples calendars" in use at our company.

Same for our business, we need better collaboration support (public folders, shared calendars etc.). 

When it comes to the collaboration and working with other calendars, even get rooms calendar become important.