What is the usefulness of the genXDocletdTags="true" setting
in ANT's middlegen's hibernate task?

What can xdoclet tags in a hbm file be used for?

Thanks.

It was added as a facility to move your development easily to XDoclet (topdown) when you are starting from a database (bottom up).
In this case, Middlegen provides a once off conversion/setup process then its use is no longer necessary (unless you have DBA staff that prefer/can-only work by creating the database first).
I would not expect that setting to be premanently on. I suppose it can also be an source of education as well.