Is Batesian Mimicry a form a parasitism? To what extent is the species with real defenses harmed by the defenceless species?
In my own opinion, I would not classify this as parasitism, as more unpalatable species are eaten with Batesian mimicry (and eating causes death). Parasitic organisms often do not kill the host, whereas in this case, it does. However, you are correct in stating that a unpalatable species with aposematic coloring is detrimented by the presence of a palatable Batesian mimic. 

Predators often recognize the aposematic coloring of an unpalatable or dangerous species. They "train" themselves not to eat that particular prey, as it is detrimental. The presence of a Batesian mimic which is palatable or not harmful, in simple terms, "untrains" predators of the harmful nature of that particular aposematic coloring. Predators are "confused", and are thus more likely to eat preys with that aposematic coloring, and results in more death of the species being mimicked, while providing protection to the species that is mimicking.

As far as I'm concerned, within an ecosystem, this phenomenon happens with any Batesian mimicry to some degree. Factors that may influence this degree is the recognizability of the aposematic coloring, as well as the ability of a predator to recognize this coloring.

I hope this clears things up.