I can't remember where I read a response from a Host why they had different tier plans, but it said (paraphrasing) that people payed more for his top tier plan because they wanted excellent service, and those that bought his discount plan KNEW they were not going to get the same level of service because they bought the least expensive plan.
I thought maybe this guy hosts on a different planet, because my experience is that no matter what the price the customer pays, they all expect (sometimes unrealistically) top shelf service and we try our best to give it to them.
However like you said, sometimes you have to put your foot down and say enough is enough, because the person is taking your time and resources from your higher paying clients.