I'm almost sure they're used in every center... I know they are in mine, usually referred to as a combination shift, article 22.3 in the contract. Limited spots though you have to bid on it. I know in my center you can count on one hand the amount of 22.3 drivers we have, they usually deliver EAM or NDA's.
Are you talking "Utility drivers"???? We have them still here (basically part timers who are available to drive when needed). Not a combination shift per article, but old fashioned Utility drivers. Usually do the overflow work or bulk stops as needed.
In my center, one of the 22.3 combo drivers delivers eams and reg nda, not any ground work. The ground work is for full time drivers. The cover driver language says they cover for absentees and vacations. If it is overflow, who are they covering for? Could there not be proper staffing that day? The contract does say the company has a obligation for proper staffing. If the company does use a cover driver on a paticular day, the preload or local sort work must be offered to a full time driver first before offering it to the cover driver, unless the cover driver work was not planned. This was a fight for a while and now it rarely happens. However, there are a few different circumstances that do come up when thay can get away with it such as injury,accident, sick, anything beyond the employers control. Im not saying I am right on all of this, just looking for a discussion. This has worked for us.