The Bennett Holiday character (played by Jeffrey Wright) looks like a very unhappy character, made worse by the annoying housemate, who is always drunk on-screen. What is that supposed to achieve, in a movie that is already full characters?
The housemate you reference is actually Bennett's father. This review from Emanuel Levy explains the significance of the relationship and also points out that there are 5 father-son relationships in the film:
Gaghan was the writer and director of the film. The other father-son pairings were Bob and Robby Barnes (Bob worries about losing his son as he goes off to college), Bryan Woodman and his two young sons (one drowns in the pool), Emir Hamad Al-Subaai and his two sons, Prince Nasir and Prince Meshal (the older son is in line for the throne, but is too reform-minded), and Saleem Ahmed Khan and Wasim Ahmed Khan (migrant workers who dream of returning to Pakistan).