Rivet Joint, I understand exactly what you're asking in that it seems ludicrous that they would run a Heathrow - Malaga service when there are cities all over the developing world which aren't served and (pre-BMI) they were clamouring for more slots.
To answer an individual point in your posting regarding BOS-LHR services, the reason BA is increasing its services between the two cities is probably because
American Airlines is pulling off the route as of next year entirely.