Pre- Formed parties in FL drastically (negatively) affect gameplay for all other players who dont have luxury of PVP minded friends to join them in FL.Yes there are parties from FC's etc, but they would have same experience as if they would que alone.
And if the team is LUCKY when those premade PRO PVP players join their team they just steamroll everything taking away all the thrill, fun and sense and equal competition.
PVP is already hated by majority of FF14 players and completely onesided wipes and wins just fuels discontent even more.