I often use tags for organisation, but for that they would need to be able to sync with the server. Therefore, the Exchange Categories would be very helpful. Unfortunately, the existing tags are not.
If there are reasons for not supporting the Categories: MailMate offers the possibility to map tags to (standard and custom) IMAP keywords. The same could maybe also be realised in eM Client. Here is an example of my MailMate configuration:
In this case DavMail takes care of the translation of the categories to/from IMAP keywords (MailMate is IMAP-only) and these settings map the tags to/from IMAP keywords. Sounds cumbersome, but it works perfectly. (One may ask why I don’t continue to use MailMate then: DavMail does not support shared accounts.)