Angelus is pretty much a Prince of Vampires. The Master deemed as the creature he wanted at his right hand, he was a member of the Order of Aurelius he took over Spike's gang in a few minutes, all other vampires seem to both fear and respect him. Heck, even the Beast, who in demonic stature far outranks a lowly vampire, was willing to entreat with him. So, if part of your plan requires a lot of Vampires, you want the best on your side, and to choose Angelus as one of your fell paladins is a no brainer. Consider how Graduation would have gone with Angelus and Faith present.