Rohypnol makes the extremely irritating class of horse crossbowmen deadly, I'm sorry but he is more then a nuisance on the battlefield, and he is very much a threat as he always has been on any horse.
I'd much rather have him on my team then against, and he has greatly helped every team of mine when I see him play.
Like Slamz has mentioned, most of his kills are low quality, picking off wounded or peasants at the periphery, or after the round has been decided and it's in cleanup phase.
Remember the autobalancer does balance off kills and k:d, so if you end up with Rohypnol that means the other team is getting some other high killers to balance it out. So you can keep him, I think I'd rather have a guy like manowar on my team, who is gonna get the same sort of k:d but will get it by being in the thick of the fighting in the first two minutes of action when the main clash is happening. Add in the fact that when the balancer puts him on your team, you are also getting his badder, and even more useless HX banner groupies, while the other team will get some pipe hitting clan-unit instead.