If fly almost exclusively Y, because that's all that work pays for. For me, status matters. It's the difference between having a good chance of a free upgrade to Y+, and sitting in a rubbish middle seat in row 57. It's a free drink and comfortable seat in an airport lounge rather than a sticky table in a Wetherspoons airport pub.

I for one started with DC becuase I was doing quite a bit of flying to places like AMS which were served by BMI from Heathrow. The only thing that has really pushed me away from being a more regular BMI flyer is the erosion of routes, especially that one. Lufthansa have probably been the main beneficiary of Euro route erosion, rather than BA, because of my access to lounges (albeit AMS is a bad example there, but you get the point)

Being adopted into the BA scheme on good status terms would probably be a good incentive for me to favour them over KLM and Lufthansa, all other things being equal.

Quite how much they need a few more flights from a fortnightly cheapo-Y flyer who does a handful of TATLs a year, I'm not sure.