It's an accepted defence mechanism in wormhole space to anchor POSs at all the moons in your home system. Obviously this is a more appealing defence in systems with a small number of moons, but even in systems with 20 moons it's still a viable defence. At the risk of stating the obvious, the defence aspect is that anyone wanting to obtain a foothold in your wormhole system would first have to blow up an offline POS before they could anchor their own.
This is a sucky situation to have in EVE and is a result of the game mechanics not providing a mechanism by which to unanchor POSs belonging to others. This is a topic that comes up from time to time in my corp and my answer is always the same: It should be possible to using hacking modules to force an offline POS into an unanchored state. This could be either with the Codebreaker I and II modules or with a brand new module and/or skill.
Personally I would go with a requirement of a Codebreaker II and a new skill of Advanced Hacking to trigger the offlining process. There should be some alert provided to the POS owners in order to allow them to arrange a defensive fleet. There should also be a longer timescale for the unanchoring following a successful hack attack compared to the owner unanchoring the tower. Maybe a base unanchoring time of 10 hours with a bonus of 1 hour reduction per skill level in Advanced Hacking.
Such an attack should be viable in all securities of space. It would be comparable to stealing from a jetcan or looting someone else's wreck with the applicable shootable state persistent for the entire time of the unanchoring process. Cancelling the unanchoring should not be possible by any party.
The resulting situation would allow people to make ISK salvaging abandoned POSs without requiring wardecs in hisec. This would also free up many moons which are currently 'reserved' by people who may not even play any more. The defensive aspect becomes less appealing in wormholes but not entirely gone, just more risky. I can't see any downside to this plan. Can you?