Choices are unappealing-- I live in London, particularly West London, close to Heathrow, and travel to a large number of different places, both for work and hols. If going to one particular place or a couple, a competitor or different alliance might make sense, but given the range, it doesn't. Status matters-- that's why they do that-- so clustering flights on BA makes sense. A foul recent experience with the bmi call centre, and some questionable flying a year ago on Air Can just confirmed that it isn't worth fighting against the obvious choice.