There are hundreds of Alliance willing to come out for the Battle. Bringing multiple raid groups makes the most sense. Finish one phase as soon as possible then move onto next phase to get more Alliance their win.
Its a service to the Alliance as a whole. Not just exclusive to RUIN members.
100 Conquest once per week, and then 50 conquest once every 2-4 hours?
Incursions can be done easily without bringing in a raid group to control the zone.
BG’s and Arena would be exclusive content that only 40 or 2-3 players can come out for. RUIN does do the former though. Just not a high priority.