Having followed all of this I really don't understand what it is all about.
As you say to outsiders this has no logic. But I suspect some in BASSA see picking a fight with WW as unfinished business from the 2007 strike that was called off at the last minute after WW did a deal with Tony Woodley. There was a lot of anger at this in the BASSA camp and I suspect that some have been spoiling for a fight ever since.
Also, Mixed Fleet heralds the beginning of the end for BASSA (if it hasn't happened already). Mixed Fleet will have no seniority and fewer supervisory positions. So, it is likely to have a much higher level of staff turnover and staff are likely to feel less need to join a union if they don't expect to be in the job for more than a few years. Add to the that the fact that a separate body of crew at LHR forming a separate bargaining unit means that BASSA's negotiating strength is going to weaken considerably.
When Mixed Fleet starts, I await with interest to see how BASSA responds, ie whether BASSA try to undermine Mixed Fleet crew and leave them out in the cold or bring them onside and try and stir them up.