Green slips trigger a increase in required manning. Every airline needs a mechanism to cover peaks in flying. Our GS system is one of the most punitive to the company in the industry. In fact reducing GS's is the companies latest focus in crew Skeds.
OOB pickups would be wide open to abuse, smooth manning between bases for the company and reduce jobs. If there is a loophole or way to abuse a system we have pilots who will exploit it. Witness the current mess with TOE trips as a example.