I find the reinforcements rules very disappointing and as they stand I would never, ever use them in my games.
The basic philosophy of any game I have ever run is "the PCs are the heroes of the story." They might not be the most skilled or powerful people in the story, but they are the heroes. The whole concept behind the Reinforcements rules throws that out. It is little more than a player initiated Deus Ex Machina, completely removing the PCs from the adventure and handing things over to some super powerful NPC who swings in an saves the day. The fact that the NPC happens to be controlled by one of the PCs doesn't change in fundamental insult of it one bit, IMO.
Now, when I first heard of there would be something called Reinforcements in DH2, I loved my (mistaken) idea of what it would be. My thought was "a Battlefleet Scarus fleet action." For those not up on their Eisenhorn, the quote refers to the sudden arrival - at an Inquisitors request - of a small army of Imperial Guard, Naval troops and (IIRC) orbital support from a Navy Frigate... all guided and directed by a main character. To my mind, that's what the Reinforcements should be... the application of Inquistorial authority to requisition assets from loyal Imperial agencies. It should be the ability call in an Arbites riot team, or a Battle Sister purification squad... up to and including orbital support from Imperial Navy ships... but the PCs remain the main characters, the leaders and controlling force behind these powerful assets. Not the players, the PCs.