First of all: I'm not a defender of the leecher or let's better say bad mods.
A leecher is just someone who uploads less than he downloads. This is not a matter of the client you use, an unmodified aMule does not differ from any random eMule-mod in this purpose. It's just depending on your upload speed and the download you receive. This is easy to get by downloading many files at ones.
Now about the bad mods: A bad mod acts againts the rules created by the devs of eMule. These are made to ensure fairness in sharing. Things like friendslots, community sharing, manual kick/ban or manual reask do either hurt the network due to increased overhead or they are not fair to other clients. The creator of bad mods want some features, that do not comply with the rules, mostly for getting more 'comfort'. But like the edonkey 'horde' could this be something that increases your personal speed because you do a sort of filetrading inside a community (this happens with all mayor web-communities, they post a release and everyone is asking for a friendslot or better: people are exchanging friend slots, it just gets worse with an automatic community, where all member clients favorize each other in download)
If you use a clean client your possible speed is lower through this, but you're (theoreticly) protected from beeing leeched by the credit system. If the ratio is low it's not very likely that you upload again to this client, you're not a valueable source as long as he didn't upload to you again.
Real leecher clients get their speed mostly by pretending to be a member of a community (they will be favorized in upload by the other community members, but they wont upload to this community) and by using the creditsystem. They upload less than a chunk to you and try to get a full chunk by you. If you're a valueable uploader, you'll be 'feed' to get more upload, if not they didn't 'waste' a full chunk on a useless source.
As I hope to have pointed out, it's not really needed or doable to ban all these bad clients. If someone uploads less than a chunk to you he's not automatically a bad user.
Of course you could try to detect communities or bad mods with thousands of lines and you may feel better if your clients tells you, that he banned a shitload of leecher. But imho is every leecher a possible source where I could get some download and where's the use if I need a rare file with 2 sources, which are both banned as a leecher?
I'll compare it with a firewall, that sounds a big alarm, 'cause it had detected a port scan and successful defeated the bad attacker. Maybe it had been the preparation for an attack, but more likely it was just harmless.
Hell, I start writing to much about unimportant topics.
Just one little remark at the end: All really bad clients (those who break with the GPL and maybe even are commercial -> emcrypt, spread corrupt data or do not upload at all/limited) should be banned without discussion.
