Seniority List Protection
Stinker99, you ask for a valid reason why OS pilots should not be on the BA master seniority list.
Why would BA be against this? Seems simple enough with no obvious costs implications (at least until OS pilots get to bid onto Mainline fleets).
Perhaps we should ask why does the BACC (supported supinely by BALPA) feel that the seniority list affords the protection they seek from:
1. undermining Mainline T&Cs
2. off-shoring "their" flying work
3. Growing the OS business by shrinking LHR operations
4. Diverting (as yet unplanned and unknown) investment beyond what has already been identified to OS as the more likely to generate greater longer term returns.
Because if, as I suspect, the plan is that the OS CC will be a BACC puppet and will work towards undermining the lower cost base and squeezing on flexibility over time to the extent that you will eventually be unable to see the join, I think you have your answer on BA's motive.