Why should the directory of one user not be generally accessible to other users (not even for read-only access)?

What will be an ideal response?

The
knowledge
of
the
existence
of
certain
objects
should
not
be
available
to
unauthorized
users.
For
example,
a
knowledge
that
Jones
and
Smith
are
working
together
on
a
project
(as
evidenced
by
shared
access
to
many
files)
might
be
sensitive.
Sometimes,
testing
or
development
versions
of
hardware
components
or
software
systems
should
not
be
known
by
all
users.