Although I admit that pre- and post- feed weights have limited value, I
believe they do have value.

For example, some pediatricians and nurses insist on giving bottles of
breast milk or formula to a premature baby after a breast feeding to be
sure he gets enough.  By weighing the baby before and after a feed, the
nurse can decide whether or not additional milk needs to be given.  If
the baby usually takes 30 ml. and he only obtained 5 from the breast, he
needs to be supplemented; however, if he already got 45, he certainly
does not need to be.  Paula Meier has described this technique as useful
for parents after discharge of their premature infants and I have
personally found it to be so.

I have also found this technique useful when managing a low supply.  It
helps the physician and LC to decide how much supplement is needed in
order to meet the baby's needs without further compromising supply.

Also, many mothers with oversupply misread what is happening as
undersupply.  The scales reassure them and make it easier to them to
listen to your management suggestions for oversupply.

I think pre- and  post- feed weights are just another management tool,
like shields or supplementers, bottles, or cups, which can be useful
when used appropriately by a trained professional or a parent who has
been instructed in their use.