The main reason to ally with them is the fact that even though they're at war with the Imperium, they actually<em>like</em>humanity, and deeply want to be their friends.  Hence you can bugger lots of really advanced technology out of them before killing them for trying to brainwash you into forming some kind of "federation" to peacefully cooperate with other races to explore the starts in peace and harmony.<br/><br/>Also, to give you a clearer idea of what your working with, Christopher, here's some pictures of a Crassus armored transport:<br/><br/>(http://www.forgeworld.co.uk/Images/Product/AlternativeFW/xlarge/crassus6.jpg)<br/><br/>(http://www.forgeworld.co.uk/Images/Product/AlternativeFW/xlarge/crassus2.jpg)<br/><br/><br/>The Brutus has had two additional decks added to its top, as well as an extra bolter turret on the back at the second level, where Darius is.  Four of the hunter killer missiles are located on the top deck, and another two are bolted to the top of the heavy bolter sponsons.