The fact that the calculation status (manual or automatic) of an excel
workbook you open is altered if you already have one open with a different
calculation status seems to have no advantages and several disadvantages.
Inexperienced/occasional excel users may not notice the change and suffer as
a consequence. I have experienced this when such people have input data to
auto-calculate templates I've sent them but obviously had a manual calc
template open beforehand - and been highly embarrassed and apologetic sending
their input to me apparently not adding up. The fact that I could fix it with
one touch of F9 didn't reduce their discomfort. The present process is
deliberate per your "support" site and has been through several versions of
excel - but I cannot see any advantage in it, only problems.