It irritates the crap out of me when people say that. I was told "just pump" more than once, even after I completely stopped responding to a pump. Even while painfully engorged after a long day away form baby (twice), I didn't produce more than about three drops after a half hour of pumping.
The average output when using a pump is not enough to feed a baby. Some people are fortunate enough to be able to pump exclusively, but most aren't. It's also ridiculous to take issue with the fact that someone doesn't want to deal with all of the clean up that comes with a pump! "Just pump". smh