Well I'll have a go at explaining it. When BA wanted to merge CFE, CFE brought a fleet of aircraft of comparable size to many of BAs (737-500 springs to mind), plus some ATRs. That means CFE would be flying aircraft which were roughly equivalent and interchangeable, and BA management wanted to fly them interchangeably on BA routes ex-LGW. Thats is why the BACC insisted that CFE people were brought onto our seniority list, and with that comes the BA T & Cs.
The BACE deal is entirely different. The BACE deal does not bring a fleet of comparable aircraft to fly interchangeably with ours. The BACE deal sought firstly to boot all BA pilots out of the regions, then to transfer an entire fleet of aircraft out of BA to be flown exclusively by pilots of a subsidiary and hand over all the routes to that subsidiary. In short, the intention of the deal was to enure there was no further BA presence in the regions. In this instance the BACCs first priority must be to look after the interests of the BAR pilots who wish to stay in the regions and the ex-CFE who wish to stay on the type. They have gone some way to acheiving this (despite howls of protests from BACE on this thread).
On the subject of merging or somehow incorporating BACE on the BA T&C, then I suspect this could only be done for those who fly the RJ as (ATR excepted) BACE don't fly any type which is of similar size to, our interchangeable with, any BA type. Furthermore, universal incorporation into the BA seniority list (which would be at the bottom) would undoubtedly lead, after an initial protected period, to quite a few mainline BA crew bidding onto regional bases at the expense of your people who would be junior. I suspect this would not go down well at BACE where most FOs would see regional command prospects disappear for some time. So, as far as I see, the only place where we could reasonably merge seniority/ T & Cs is on the RJ, which is effectively an entirely new fleet. This then brings into question the BACE contracts, as if you fly an RJ on BA T&Cs you'd probably have to go onto a BA contract, which again means going to the bottom of the seniority list and retiring at 55. Also, if for any reason the RJ left the regions to be replaced by a yet smaller aircraft, which was below whatever level of seats scope decides on, then you could look forward to be directed down to London as a junior FO.
In my opinion a way for this situation to have been handled better would be for the RJs to swap to the regions on BAR T&Cs, those who wanted to fly them can stay and those who don't (which is many) leave, leaving the regions short of crews. The gap is then filled by internal recruitment from subsidiarys of those who wish to work on BA T&Cs but on the BA seniority list with the associated down sides. Unfortunately it is not possible to press for BA T & Cs whilst there is no intention to merge with the BA seniority list, and from the opinons expressed on this forum it seems that there is little interest in doing that from most BACE pilots.