OK, I think this is the point of confusion. If one uses Casper to clone a
source drive with say 3 partitions over to a destination drive, it is a bit
inaccurate to say "you can clone those three INTO an existing partition on
the destination drive".

Because what is really happening must be this: when Casper (or any such
program) starts cloning TO the currently existing partition on the
destination drive, that existing partition is destroyed (marked as
unallocated, behind the scenes), in preparation for the cloning of the 3
partitions.