Noted, but I do believe after hundreds of years, and the evolution of Sirius, a faction that prides itself in intimidation and protection of their borders would have a capable flagship for final defense.
This would be -one- above GB class capital ship, and wouldn't be heavily used or abused.
A GB as a flagship for such a powerful organization just seems very very out of character for the GMG. It's like the Mollies slapping a "New Dublin" sticker on a Rogue GB and calling it their flagship.
EDIT: Also, take a gander in S-13. We've got OC Destroyers in system left and right. IMG and Explorer GBs would be utterly slaughtered without bomber support.