Does this spells the end of leechers?

Sounds good to me. Simple and logical.
You take half the monster's HP off - you get half its EXP.
You take 0.0001% off - you get 0.0001% of the EXP (regardless of where the other party members went - the end of BF leeching).
You hang on a rope - you get nada (an end to afk leeching).

Parties are for party buffs and ease of communication. Not for getting experience off others' kills. (What about meso?)

Some balancing still needs to be done.
I'd like to see the power of AM non-Ultimate skills increase, so they can train with them at the same speed as other classes.
I think clerics would need to gain more EXP for healing their party-mates, to make up for the cut of the kills they won't be getting.

There would still be isolated cases of injustice to characters playing a critical supporting role in a PQ or other party situations. Extreme case in point: HT Seduce Mule. But I think these individual cases will have to be handled by the parties themselves. I can't think of a "formula" Nexon can implement, that would not be subject to abuse.