Why was the game built around guildleves?

[Rowyne]

8players: would need to plan out and a good class set to win and times to do.
4players: Don't need much planing it's a go in fight get out type of deal. healer,dps,dps,dps would do in this type of event.

I was thinking 4-mans would be more like Tank, Healer, 2 DPS