Yes,
That's true that the directory structure is backed up, however there is
no way as Juan states to restore the empty directory.  The only way
I've found to find out what directories may have been empty during
backup and hence can't get restored during recovery, is to issue a Q B
* -dirs -sub=yes  command, and it will give you a list of all  the
directories that have been backed up.  It gets very tedious to pick out
the ones that don't exist (didn't get restored), but if your ambitious,
you could come up with a script to create (mkdir) the ones that you've
found didn't get backed up.   I've had this problem with the few full
system recoveries that we've had and the system people have tried to
circumvent the problem by placing dummy files in these 'temp'
directories that may be empty from time to time.  But I still agree
that there needs to be a better way to go about addressing this problem.

Larry Abbinante
labbinante AT ual DOT com