Another point of view:
If the data structure is well defined and it will not change (or it will change when new release occurs), sql database can fit the data structure very well.
But, if the character of the data structure is, for instance, "we know that the tree have following base structure, and some nodes could be enhanced by some data from time to time, some layer keeping generic xml structure could be better than never-ending story of changing definitions of sql tables. In this case, you can choose eXist or implement xml layer in RDBMS.