The main problem of 5th columning comes from a few factors:
1. It is very hard to directly attack a power. All you can do is undermining or opposing expansions. Both are easily countered, and both are easily cured. 5th columning offers a way to attack a power in a lasting way, causing more damage per time-unit invested than anything else.
2. There is no easy way to cure the damage done by 5th columning. If a 5th column tries to prepare a bad system for you, you are stuck with it...
3. you cannot stop a bad expansion outside of 5th columning yourself ...
Well said Flin, I agree with your points wholeheartedly. All I can add to the discussion is an emotional point that was not discussed, and a small proposed 'damage control' to 5th columning work.
1-From an emotional point of view. Malicious 5th column work to prep bad systems is 'A Bad Thing', in capital letters, in that it makes the game environment more toxic, frustrates fair-playing players to no end. I would hope folks stop considering this 'a strategy that works' and consider how similar it is to causing infighting, to give you a better idea of what I mean I know a few EVE players that don't want ED to turn into a 'toxic pvp with mistrust and spying on top' kind of place like they felt EVE became. So yah, emotional point here: Having an effective strategy to undermine another power from within is Bad, big time. Makes the game less pleasant. From a design point of view it should not be considered 'interesting', it should be condemned and prevented utterly.
By comparison, Merit grinders overfortifying places I feel is 'not a big deal', this is a sandbox game and we should let people play the way they want to. So I'm rather in favor of informing the players but letting them have their merits. That little warning box stating a system is fully fortified sounds lovely. Bonus merits to players plying long routes sounds lovely too, use a carrot and not a stick wisdom. Benign actions aren't 5th columning even, I propose we focus more on stopping malicious 5th columnists.
2-Ways to stop/counter/damage-control actual malicious 5th column are needed: I read good solutions in this thread about using votes to counter bad expansions, but we also need a way to remove the 'damage done', aka bad systems that were prepped. It's not much, but I have this wee proposal:
During turmoil, remove systems with low profit instead of systems with high cost.
Malicious fifth columnists actively try for low to negative profit systems. If turmoil removes those, well, a power would lose more systems yes, but it would in a sweep remove several system of a 5th column's hard work. Wouldn't it? Sometimes a system with high cost still has an excellent profit and is a keeper, from a power's perspective.
Also, kudos to the OP for a constructive thread against a serious issue.