Agents would become the new information bottleneck, narrowing the otherwise delightfully anarchic infobahn, which was supposed to replace the broadcast model with something more inclusive. An agent's model of what you are interested in will be a cartoon model, and you will see a cartoon version of the world through the agent's eyes. It is therefore a self-reinforcing model. Now true agent believers can answer any specific criticism by postulating better agents. But the specific problems are not the ones that make my blood boil, anyway.
Agents make people redefine themselves into lesser beings.
THAT is the monster problem. I don't think so. You change yourself in order to make the agent look smart. Specifically, you make yourself dumb. Well, actually, agent programs as a rule will also have worse user interfaces than non-agent programs.