If you read the current language unassigned bid routes can have a bid driver on them, but are essentially "(swing drivers : FT non bid drivers)" as management can move them as they see fit, even for training or coverage issues. The reason, at least in our center, why these drivers tend to not get moved is they are generally the highest seniority swing drivers. Bumping them upsets the Yen and Yang and generally isn't worth it. The new language is meant to prevent them moving them for any reason, just for "training for 30" days. The is also meant to cut down on the 15% to 10% unassigned route fiasco that goes on, there's no reason to have that many routes unassigned besides the fact management can toss whoever they want on them.
Short version
The part where "Package car drivers not assigned bid routes cover for vacations, sick days ect is the reasoning behind this change. Before, those drivers that "bid" unassigned routes" counted towards this, in the next contract, they don't.