I'am not sure i follow you here this just makes no sense to me. It makes sense if that defender can't see the attacker but if it can see the attacker it can also defend.
This is how it's always been. The invisible 'defender' has always not been able to stage a tile defense as the game will assume 2 things:
1) the defender will want to remain invisible and unseen.
2) it would be unfair to both the defender (potentially) and the attacker since neither side has made any determination as to the odds of the battle whether battle would be wise. (Let's say you're moving a visible unit onto a tile that has an enemy unit there you cannot see... it would be quite a surprise to be entering battle unexpectedly without a help hover there to indicate it. If this wasn't as unfair to the defender as well then it would probably be fine but the game assumes the unit with the upper hand of invisibility would want to declare the fight on their own terms. Otherwise you could go hunting for the unseen by simply fanning out and poking your attack units around until they hit the unseen target.)
In vanilla the invisible defender would be shunted off to another tile since they could not share the tile with the visible enemy moving in.
C2C allows the visible enemy to move onto the same plot without forcing the invisible 'defender' to move - allows them to then share the plot. C2C then also now allows the defender to then attack the visible enemy 'stack' with an Ambush command.
In vanilla, the same style of frustration about guarding particular tiles would be most apparent with the introduction of stealth naval ships. Let's say you had a Destroyer guarding a naval resource like fish. You then upgrade the destroyer to a stealth destroyer and all of a sudden they no longer guard the tile against an enemy sub that cannot see the destroyer. The sub could then move onto the tile, forcing the invisible defending stealth destroyer to move off the tile, and raze the fishing nets before you could then retaliate. The same would be true if your Stealth Destroyer was guarding a stack of vulnerable work boats for example. The work boats would be captured without a fight because the attacker couldn't see the defending Stealth Destroyer.
Since the same frustration CAN exist with much greater frequency in Hide and Seek, the Standout status makes it possible to ensure your unit is visible, regardless of almost all ways the unit may normally be capable of being invisible. (I say 'almost' but the truth is at the moment it is actually ALL ways - there are future futuristic methods of invisibility to be further developed that will be based more on other statuses to come.)
With the recent AI advancements in Status use, this should no longer be a critical issue for the AI - though I do realize that it would be helpful to further develop this so that if a unit is set to specifically protect a vulnerable unit then it would value standout but the current more sweeping assumptions works for now.
packrat! 
