If it's an either/or Bose then yes I agree. But I can't see at Bournemouth anyway why there isn't room for both? At Southampton there's as issue of Apron space.
Surely with mainly fixed costs it's a case of more revenue by having BOTH interests served. Unless GA was so prevelant that it delayed CAT movements. But as you say Bose.. there's not really much reason to go to Bournemouth apart from IR training, which I assume is booked as a slot so avoiding delaying Ryanair et al.
Ultimatly, it's a free market. If I need to go to Bournemouth I'll go and I'll pay. I won't pop in for the sake of it. At least I'm free to make my own decision.