Groundloop - Yes but what yeilds do they get at Bournemouth?
For the only easy service from the area I very much doubt the yield would be much different between BOH and SOU. Do people who fly from SOU have more money and are willing to pay more?
Anyway, concentrating on "yield" is not the best way for an airline to make money - the total "revenue" from the flight is more important. Yield management and revenue management are not the same thing.
Plus with 20 more pax you have 20 more people to spend on board.