Should "su" change the USER environment variable?

[v.wales%ucla-locus at sri-unix.UUCP]

From:            Rich Wales <v.wales at ucla-locus>

The "su" command in 4.1BSD does not change the value of the USER envi-
ronment variable.  This seems strange, especially since "su" does change
the HOME and SHELL variables to values appropriate for the new user.

(1) Can anyone think of a good reason why the USER value should not be
    changed by "su"?

    (NOTE:  I fully realize that no one should depend on the USER value
    as a reliable user identification, since anyone can change their
    USER value to anything at all.  Hence, no flames about system secur-
    ity, please!)

(2) Does "su" change the USER value in other versions of UNIX, such as
    4.2 or System V?

-- Rich Wales <wales at UCLA-LOCUS>