The way I see it, C-SPC provides a robust region, and Emacs users will
continue using this even after we implement shift-selection; holding
down the shift key is too much of a nuisance. So we're talking about
how Emacs behaves for new/casual users, who use shift-selection
because they're either unaware of or unused to C-SPC. It seems to me
that such users would expect the shift-selected region to be fleeting,
since that is the behavior in other editors. Furthermore,
shift-selection is *inherently* fleeting, since entering any unshifted
motion key deactivates the mark, and motion commands are
psychologically "tinier" (or rather less consequential) than most
commands.
I've been thinking about cases where you might want the shift-selected
region to persist after a command. The only good example I can find
is M-x eval region, which doesn't deactivate the mark in tmm. But
even in this case, the advantage either way seems to be marginal. If
you care enough about keeping the mark active, why not use C-SPC in
the first place?
Now, there is one other example, and that's switching windows. You
might argue that it's good to preserve a shift-selected region for
this, so that it is still there when you return to the window. But it
seems to me that the effects of shift-selection and mouse selection
ought to be as close to equivalent as we can make it (*), and
preserving a shift-selected region when switching windows is
counter-intuitive in the mouse selection context.