The important thing here, I think, is that we are starting from the main terminal being destructable, and the loot dropping. Those are key elements that will drive some corps to attack and the owning corp to defend.
The details now, are in balancing attacker time/size to defense time/size.
A scenerio that the 12 hour timer eleminates, is an ad-hoc coalllition of players embarking on a 3-4 hour mass terminal destruction spree.
The 12 hour timer does not eliminate an issue where a dominent organized force on the server embarks on a weekend mass terminal killing spree.
Taking into the potential for returning players bolstering a single corp or alliance to the point where it numericially exceeds the largest possible defensive forces by a significant majority, it would be likely that this force could, in a Saturday, visit all main terminals built to that date, reducing them to 60%. They could then log-in on Sunday, and complete a server-wide wipe.
Or taking a real example, if all 600 of Nex's players (not picking on you Nex, your just the best example) return with characters that were hibernating (gathering EP), this size force could indeed roll the every terminal in a single weekend.
The assumption here is that even if corps manage to get 50 players online for defense, that the turrets and other devices would not over-come 12 to 1 disadvantage; mainly because if it could, then it would take 600 players to defeat a 50 man defense.
While its basically impossible to create a game mechanic that would STOP a 600 man corp from taking ALL the terminals, it is possible to add stops to prevent it from happening in a 24-48 hour time period.
While that's an extreme example, there are a lot of current examples where 12-15 man squads rolling up to an outpost on an off-time zone could easily be greeted with only 1-2 players.
Looking at the timeing here:
0:00 15 man Squad attacks outpost defended by turrets and (3) defenders. They eliminate the defenses, reactor, and the majority of the nodes over a 3 hour time frame
03:00 Main term is reduced to 60% and goes immune.
11:00 Owning Corp's prime time
15:00 Terminal is repaired, reactor is back online, and some turrets, walls and TF are redone.
Terminal Immunity drops.
19:00 Small off time zone team has worked through the night to bring terminal defenses back to 80%
0:00 Next day, still 3 hours remaining on reinforcement, 15 man squad shows back up
2:00 Defenses defeated, this is the 'middle' of the day for off-time zone, not the middle of the night, so they are off at school or work, no CTA alarm clock defense is do-able.
15 man squad has 1 hour to bring the outpost from 100% to 0.
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So, the choices with this system are .... well, there is no choice. Once someone attacks your outpost, and pushes it into reinforcement mode, you have to evacuate because the odds are they will very likely beat the 12 hour timer and collect all your loots. It makes it that much easier, because instead of being able to rebuild in those 12 hours, your're going to need them to move.
On the other hand, if you make it even an 18 hour immunity, then you push the attackers into THEIR off timezone, and then the pendulum swings the other way, making it almost impossible to lose defense (with even a resonably sized prime-time defending presence).
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Currently, it takes at least 3 days of pressure to simply switch outpost ownership, without any other benefit to the attacker. The gamma system provides way too much incentive with dropped loot, in comparison to the time the attackers need to dedicate to achieve it.