The Autodesk support team indicated the following.
'The Datum Target symbol only accepts 4 characters as you are already aware of. The inclusion of more characters has been requested as a wish-list item for future Inventor releases, however the request is still being analyzed by the Development team.
A work-around to include more than 4 characters in the symbol is to create a Sketched Symbol with the appearance of Datum Target and include prompted entries. If you want to use that Sketched Symbol in different drawings you will have to save the symbol in a template drawing and use that template for all the drawings that require the symbol with more than 4 characters.'
They also included a vidoe and sample. It is an OK work around.
The benifit of the Datums is the cirle or rectangle that is created with the datum.