I can't understand why when Flybe took over from BA they started using this more complicated system of Flight ID's.
If you are listening on vhf airband as well as using Radarbox it's not easy to relate the Flight ID/callsign to the BE flight number - whereas before it was simple.
It's not being done for your convenience.
The issue of "callsign confusion" (two aircraft in the same bit of airspace at the same time with similar Flight IDs that could be mistaken for one another) is being taken very seriously.
ATC couldn't care less that the passengers on board the flight using "BMA3EV" as a callsign actually have "BD121" on their tickets, if it reduces the possibility of one crew following ATC instructions intended for another aircraft.
Anything that lessens the risk of two aircraft being in the same place at the same time is fine by me.