The term ‘leech’ is often used in a negative context in language. In the case of P2P file sharing, the word is still used in a negative and derogatory way. Peers that download more than they upload are called ‘leechers’. They are not looked upon favorably in the P2P community. Leechers are seen as people that take but don’t give. They are the opposites of seeders in P2P systems.
Typically, leechers will stop their upload and prevent access to their files as 
soon as they have completed their download. This, of course, is frustrating for any peers that happen to be downloading from their file. It also places extra pressure on other seeders to share.
People look upon leechers in P2P communities as freeloaders, since they are not considered to be content distributors.
Leech is a term that is used in the computer world in other ways. Its wide use, always with the same negative connotations, makes it is easier for those new to P2P terminology to be able to grasp its context. A familiarity with computing terms in general will help anyone understand P2P terminology a bit better.