I'm not really sure why the bombers would be out and about like that anyway.

The only three reasons why a plane like that would go out would be to actually bomb something, to get some flight practice (in which case why so close to the fleet) and as part of a formation display flight like in ROTJ when the Emperor arrives (which somehow doesn't seem Vader's style).

Having the fighters out makes sense as they can protect the fleet from sudden attacks.

Having shuttles and transports moving staff and supplies between the fleet ships makes sense too.

It would seem more sensible to keep the bombers in their bays until they are actually needed (during the asteroid sequence).

It's a rare instance in ESB of the effects team sticking a new model in a shot because they can.

Though I imagine they are too iconic an ESB ship to drop from the sequence especially so late in the game.