Mercernaries don\'t necessarily only conduct attacks or assasinations on a player. A mercernary is usually in most cases is person(s) that are hired to fight in war. For example:
Fireball guild wants to destroy IceCube guild. But Fireball knows its outnumbered, so Fireball guild hires the Mercernary guild (for example). The Mercernary guild will then be hired to fight the IceCube guild but since they are hired under the Fireball guild, would have to take orders from Fireball guild or they wouldn\'t be payed.
Secondly, Pk\'ing provides economic opportunities? That\'s just three minor things. Yes I admit it does provide some economic opportunities, it won\'t make a huge difference as I\'m infering you claim to have said.