They are not kicked out of the Grove, they never wanted to stay in the Grove forever. They just took refuge there when being ambushed on the road to Baldur's Gate because they had been kicked out of Elturel. They just want to stay in the Grove until the road is clear for them to travel again. (I was too slow!)
Apart from that I agree with fylimar, being captured and then staying in the dungeon as target practice isn't much of a plan - and it never was the plan. He leaves the Grove in the worst possible moment to follow Aradin and an old fancy of his (which he didn't seem to have done anything about during the last century), getting captured or gathering intel wasn't the motivation, he went with Aradin to learn about the Nightsong and and a way to lift the Shadowcurse. He tells you as much during one of your conversations after freeing him, the morning after the party iirc.
Liam is someone I would have liked to meet again too. He seems to have been the only nice guy in Aradin's lot.
Funnily enough, the guy with the locket: if you get the kid to tell, why he took the locket, that guy tells the kid to keep it and gets all emotional. So he at least seem to have a bit of a heart. He also thanks you for interfering.
And yes, Liam would have been great to meet again. Maybe even as part of the tiefling group, since I can't see him staying with Aradins group after what happened there.
I also am wondering, what happened with Abdirak, the Loviatan. It would have been interesting to see him involved in both the Ilmater murder case and the Banite part of the Absolute plot in one way or another, since Loviatar is tied to Bane and has beef with Ilmater.
