1. It's easier to stock because it takes up less room
2. As someone who has pulled meds for multiple patients, its easier to draw up the dose you need and stick it in the bag with the rest of that persons meds, rather than try to carry around 3 or so insulin pens. As long as your nurses are wiping the vial whe they draw it up and you're adhering to the 28 day expiration, you shouldn't have an IC issue. We charge by the unit in Pyxis so we don't have any patient charge issues.
We also use community vials and charge through Pyxis.
As a check and balance, when the pharmacy techs do daily Pyxis fills, they check insulin vials, expecially the ones found on the counter or on top of the Pyxis machine, for dating. They take any vial that has no date. It is replaced to the floor and charged to the floor, as well. Some think having the floor pay for an undated vial reminds them to follow procedure--I'm not so sure about that.