I usually waste when I pull from the pyxis but here's one reason some nurses may not (and this story is not the first time or pt this has happened with):
Pt was getting 0.5mg Dilaudid, which was 0.25mL. I would draw it up in the med room so I could waste right away, then take the syringe to the pt's room and administer it. 2 days into taking care of this pt, she said, "you know, I never feel it/get good pain control when you administer it, but when my night nurses do, its incredibly strong and helpful." I went over that I dilute it in NS, maybe I was pushing it slower, etc but then I told her i'd bring the unopened vial in next time and measure the dose in front of her. I did, and the pt said the same thing about not feeling it, etc even though we both saw that the correct dose was being given.
The pt was having a legitimate lot of pain, so I think the night shift nurses were giving her a "nurse's dose", possibly up to the full 2mg.
This is wrong on so many levels (call the doc so the pt can have consistent pain control since 0.5 isn't working!), so now i'm more inclined to bring the unopened vials to the pt's room and draw it up in front of them.