A number of API clients were originally written with a "core" component that deals with metadata (for use by the metadata wrangler and the circulation manager) and a "circulation" component that deals with patron activity (for use by the circulation manager only).

But it turns out Overdrive is the only vendor API that the metadata wrangler can do anything useful with. For the other API clients it would be better for legibility if all the code was in one place, in circulation.