When one side gets 200 silithyst, they get a buff that gives them +25% bonus rep with Cenarion Circle.  Unfortunately this doesn't apply to quests, only to monster kills. It is worthwhile to get it before going into AQ though, and a raid can collect that much silithyst in no time.

Oh yeah, it also gives you a 30 minute buff that increases your ranged, melee and spell damage by 5%.