When I started as an airline passenger there seemed to be more examples of flights picking up passengers at a couple of airports before continuing with the "trunk" haul. Apologies for terminology.
Thus I can recall lurching between Bristol and Cardiff in a Capital Shorts 360 before proceeding north. Similarly an Air UK F27 between Glasgow and Edinburgh before proceeding south. I assumed the extra cost of serving the second airport (and annoyance to passengers joining at the first) was justified by the extra income from the second market.
It seems to me that BM want some city pairs that are too thin to be attractive to the likes of Ryan/Easy etc but might have sufficient business traffic of the type that would be prepared to pay fares that give a good yield for a direct service. Putting a couple of these markets on one flight, as in the examples above might then give a viable load.
A service was tried between East Midlands and Frankfurt. I wondered why they didn't start/extend some of the flights from/to Leeds. Similarly if serving Bristol could they not have extended to Cardiff or called in at Southampton?
If they are going to try to restart Newcastle-Brussels why not pick up at Leeds. I accept there is a cost to this and a disincentive for Newcastle passengers, but if you can pick up another 15-20 passengers, many on public sector inc. EU expenses.....