I'm not convinced that yield is better at city airport, nor do I believe it is hugely more attractive for the passenger as an airport, granted i like flying from there myself.
Re yield, I point to 2 examples.
1. In the 1990s when Bmi and BA served BFS LHR Bmi group put Manx Airlines on 4 daily BHD LHR. It was said to have done well though when ORY opened up Bmi took the 4 daily slots back from Manx in order to fly LHR ORY. If BHD was better at that time why didn't they split the schedule between the 2 BFS airports? Or move completely? I realise they did both of these actions later on but granted when BA pulled out. I believe the deal from BHD was and is so attractive that bmi stayed, not necessarily about yield.
2. Easyjet would have kept Luton at BHD if the yield was better.