I understand at least some of the reasoning behind the fight for "killing blow".
1) The old system basically gave one or two players final blow every time. The new system randomizes it much more.
2) It encourages cooperation between guilds. I played on beta and one player insisted on attacking others while everyone fought baal/cerberus. When one guild with a handful of players dominate all opposition, then the weaker players leave the game.
3) Fewer players attacking baal or cerberus extends the end period where the "final blow" lands.
4) Fewer players on the servers brings less excitement to the game and less income for the game company.

Thus, one factor in killing baal or cerberus involves having enough players interested in the game. I kicked someone from my guild on 86 for attacking smaller players in other guilds fighting our baal/cerberus. Keeping players in other guilds active pays off in the end.