I think there would be a lot of legal issues, having to do with screening of the donors (like you do when you pay for blood donations, etc). With cows, there is a certain amount of regulation regarding what they ingest, so those of us who drink cow's milk don't get medications and (hopefully!) extra hormones or whatever. I just can't see it being possible to regulate. Plus, hard to get the amounts necessary, and I definitely think that milk banks, preemies, would be much better recipients of the extra milk, as already suggested/
That said...in terms of the "ick" factor....well, if this became a common practice, maybe it'd be less icky...