To set the grumbling at ease regarding bringing a baneblade in a Space Marine army...
The Fellblade is a pre-Heresy vehicle which is identical in more or less every way to a more "modern" Baneblade. The overriding difference between the Fellblade and Baneblade was that the Fellblade incoroprates the same Machine Spirit technology found in Land Raiders. Fellblades were used by the Legionnes Astartes, and crewed by Space Marines. It's possible (and quite likely) some such vehicles have survived, at least in First and Second-Founding Chapters. So, the *potential* is there. You just need to have the right Chapter.
If that doesn't sell you, that's fine. Imperial armies are conglomerates of several organizations, and can include elements from several Space Marine Chapters who have contributed strike forces, Sisters of Battle, an Inquisitor and his retinue, and one or more Imperial Guard regiments. You could pretty easily write off the Baneblade as part of the Imperial Guard component of the combined army-- Apocalypse doesn't cover the entire battle, necessarily-- just a higher points game. A game is meant to be a section of a greater conflict, after all, and not the engagement in its entirety.
Should you have Imperial Guard infantry supporting the Baneblade? Probably. Does it still make sense to have a Baneblade in an Apocalypse army that's predominantly Space Marines? Yep.