For making a rabbet (with the grain), the blade has to stick out beyond the side of the plane a small amount for clearance. If you had a perfectly flat side that was perfectly aligned with the blade there would be trouble as soon as the corner of the blade (which is somewhat vulnerable) dulls.
For making a cross grain rabbet there must be clearance between the spur and the blade so the blade does not rub against the side wall of the rabbet. The blade can be a bit in from the spur, but the spur must be outermost.
So for with the grain blade outermost; for cross grain spur outermost.