Benzos are apparently added to reduce some of the neurotoxic effects of Ketamine, if you believe this guy:
http://leda.lycaeum.org/?ID=9299 (http://leda.lycaeum.org/?ID=9299)
He seems a little like he's trying to discourage K use, while simultaneously pushing 'traditional' psychedelics. But swim sure does like the stuff. It's true that it can be quite addicting to some. 8)