Probably driven a lot more by legislation these days, particularly when promoted positions are up for grabs. Until airline management can be trusted to run fair appraisal and selection systems (not based on fuel tables, golf scores and boys club membership) then seniority lists are probably the best option, but they are a little shaky and open to being challenged.
Its sort of like saying that no HBOS employees are allowed to apply for any new and more senior positions in the new combined Lloyds TSB HBOS group because they are "too junior".
Slotting a few CFE people in to the Mainline lists in continuous serice DoJ postion would probably make very little difference to anyone's prospects anyway. Most of them probably wouldn't move to London unless they were pushed.