Well, as nearly as I follow, I have to disagree. When you use a jointer to
make the edge perpendicular to the face, its perpendicularity depends on the
flatness of the face riding against the fence. Upon this depends the entire
joint, regardless of external help. Out of perpendicular, unless you're
lucky enough to find a complementary error, is a gap.

I am also not an "alternate" smile type on glueups. I'm a sap to sap, heart
to heart best match type, and, sadly one of the few who would agree with you
that careful thickness planing can take minor defects like crown (cup). It
is, however, incapable of taking out twist or bow. For that you need the
jointer or plane.

"GerryG" wrote in message
...
Agreed, but consider the mechanics here when joining at right angles
against
the face. If the face board is long related to its width, it'll probably
not
stay exactly flat, but will be drawn against the edge of the connected
board.
It's when the face board is short and wide that it needs to be really
flat, as
it's then difficult to draw it in with fasteners. While a thickness planer
is
generally thought to be only making the two faces parallel, it also makes
it
flat across the width, and will also make it reasonably flat across the
length
of the bed. It's those last two items that make it more useful for many
projects, especially when alternating faces as you suggested.
GerryG