Default expression only allowed for "in" parameters - why?

["Smark" <not.provided@all.com>]

LRM 6.1(19):
    A default_expression is only allowed in a parameter_specification
    for a formal parameter of mode in.

I recently had a situation in which it was desirable for me to have
a default value for an "out" parameter of a function.  It was something
like:

procedure Do_Something (Data   : in Some_Type;
                        Status : out Status_Type);

Normally, Status = Ok.  It seemed to me that, rather than setting
Status := Ok at the beginning of my procedure, it would make sense to
just set the default value in the procedure spec.  That is:

procedure Do_Something (Data   : in Some_Type;
                        Status : out Status_Type := Ok);

It also seemed like a strange idea, because I never recalled seeing
it done.  Sure enough, it is prohibited by the LRM.  What is the
rationale for this?

TIA,
Mark