Looks like an open ended answer.

Anyway, this is the theory behind why they shouldnt do it unless 'necessary'.

Joe Smith works at the post office counter. Behind him he has two mail sacks, one for mail that has been hand cancelled, the other for everything else.

Joes friend Jim Brown sends and receives 50 items a day. He re-uses all his incoming bubble mailers, leaving the previously cancelled stamps on them. He does not attach any new stamps. He takes his 50 out-going items to the post office and gives them to his friend Joe (along with a $5 bill ).
Joe drops the items into the hand cancelled mail sack, making it appear he has just cancelled them.