I apologise for dragging up your previous post but you've amended your original statement and still got it wrong.
effectively making it not worth the union subs as you are not going to get any benefit from joining.
Additionally as BA have promised that it will take years and years for MF to grow to any substantial size, you are never going to get any collective negotiating agreements as you will only be a small part of the workforce
Regardless of whether a collective bargaining agreement is in place or not, being a member of a union does bring benefits - legal advice and representation in employment matters for example.
Moving on to your second point, the number of crew employed on MF contracts compared to existing crew is irrelevant - MF and existing crew would be separate bargaining units and independent of each other. What this means in practice is that if a number of MF crew belong to a union they can ask that union to approach BA and ask to represent the MF crew as a whole. BA can choose to do so voluntarily, there is no reason to doubt that they would given that they have already expressed a preference for UNITE to represent MF crew, or through statutory means. Either way it would not affect the existing recognition agreements with UNITE branches or their collective agreements.