I'm not an expert on Cannibalism, but from what I've read, most who engage in it, do so because they believe that they are gaining the strength of the enemy by eating him. There may be other things, such as making him their slave forever in the Happy Hunting Grounds or some such. But that seems to be a common thread amongst cannibals.
As to the intoxication, that may not be just amongst the Tonks. Although it is fiction, the movie Ravenous, coincidentally set in roughly the same time-frame as this fight, portrays similar behavior changes due to consumption of human flesh.
