Yes, I think you're absolutely right, verc! And from the Gauls point-of-view, as that's what I've usually played, I like and expect to hear that from anyone trapped in my traps, except I have often beat them to the messaging. I use the trapping of someones troops more to break the ice and start a dialog with them than anything else. I offer their release in exchange for an agreement to cease raiding, or something else appropriate. If the person is not agreeable, or send a more aggressive version of your message (a rude threat), I just build more traps, and when they come to release them, I release them first, and now trap their rescue party. It can make things more expensive for them to rescue than to dissolve the troops. But a trap that is full is of no benefit to the Gaul, so every effort should be made to use their release to benefit relations, as you need to release them sooner or later (probably sooner) anyway.
I have sent troops home after a short stay with a message like "Your troops are on their way home, and have been treated well, no waterboarding, no sleep deprivation, and some villagers were sad to see their new acquaintances go."
and "Your troops should be arriving soon, with tales of how beautiful and peaceful X village is, and why it is a mistake attacking such a wonderful place." All just to see if they are friendly players and would like to talk. Some do, some are just mean and angry no matter what you say.
I have seen Gauls try to extort players with the holding of their troops, but I don't recommend that. I only took a ransom once, because some guy sent his hero, alone, to my well established capital. I have no idea what he was thinking. He offered the amount of resources he sent me to get him back.
I recently had a player try to extort one of our newest members in s4, I advised her not to pay (he wanted a continuing amount of resources over time). While we were discussing it, he decided to up the demand, so she was going to go beat the guy down and get them back herself, but as the party diplomat, I had her wait, while I IGM'd the guy that I was now involved, layed out the options for him, and told him his decision would decide future relations between our alliances. He immediately released the troops, apologized, said he got carried away.