Well here's another of my Marshal's "Mordax", yes I have more than one, in fact I have 5. The reason is simple really, sometimes I want them to have different weapons and when playing apoc I want to be able to field the Masters of the Chapters formation. In Apoc he acts as the "Master of the Fleet", hence the blue robe, and joins the other masters with "Helbrecht". I have to admit one of the best reasons for getting the Master's box set is just for the models (which look GREAT!) and because, in my case I can field 5 different Marshal's each kitted out as the mission needs.
As for the Apoc formation, I love using my BT in Apoc working in conjunction with my Guard. Between the two it's a force of around 13K points, but the BT are the Alpha dogs. I'll usually DS/Drop Pod/Outflank these guys behind the enemy lines and wreck Havoc. With the addition of the special rules gained by the formation you can really make a force to deal with.
Well, till next time enjoy