He's still an archdruid upholding the balance of nature and like any druid wants to see the aberration of the absolute and mindflayer plots dealt with. He does not lose a purpose when he has many.
The problem is, they just didn't give him enough hooks to the story in terms of NPC interactions. The main interactions are right before he scolds Kagha and sets things in order for the grove. Those are the only characters besides Thaniel and Artur that interact with him directly. And Thaniel and Artur have a very downsized role. The Harpers on top are wiped out in Act 3, so both him and Jaheira get important NPC hooks removed, but at least Jaheira has Minsc to fall back on as well as being steeped in Baldurian personalities.
We just needed more of the Grove showing up in the later acts and Halsin would have had the interactivity he needed. The shadow druids and the hag covens are a huge missed opportunity to involve him in subplots.
