Its hardly a pathetic use of a slot when you consider that DUB is one of their most profitable routes!
There was a big gap in their schedules which is now filled.
When bmi do eventually get around to building a decent long haul schedule from LHR they are going to need a decent amount of feeder flights. Regular flights on their most important routes are what are needed.
I dont think bmi would do to well if they relied soley on O&D traffic for long haul LHR ops.