I reas an article around the time Aer Lingus joined One World in which their Head of Marketing said that there was no way the BA was EVER going to get its hands on the Aer Lingus FF database.

If you looked at recent movements (SQ and DL used to be best pals, KL and AZ etc) a partner one day can be a competitor the next and if they know who all your best customers are.........