I'd be interested to know why it's called "imperfect".

Short answer is that the Roman grammarians called it that: imperfectus as opposed to the other past tense the perfectus. And, Faldo is right. The ancient grammarians did not have a grammatical category called aspect (German Aktionsart, Russian вид vid) which "defines the temporal flow (or lack thereof) in the described event or state. In English, for example, the present tense sentences "I swim" and "I am swimming" differ in aspect (the first sentence is in what is called the habitual aspect, and the second is in what is called the progressive, or continuous, aspect). The related concept of tense or the temporal situation indicated by an utterance, is typically distinguished from aspect." (link)

Ceci n'est pas un seing.