The first time you see Zevlor and Wyll protect the Grove and no one else even tries to protect the Grove too busy doing the Ritual. Even when you convince Kagha to stop the Ritual after exposing her. They still hide in their caves.
Forgot to mention if you never recruit him Wyll just stands at the gate and refuses to defend the Grove a second time. Out of Character.
The only time I see the Druids show up is when you side with the Goblins but to defend their gates nah they will never bother with gate defense even when the Goblins are outside and the Gates gives them a defenders advantage.
Unfortunately this is MEANT to be - even if you redeem Kahga, all the people that were following her to the Shadow Druids actually still complain at you for stepping into their business, saying that you haven't "swayed them" or that you "shouldn't be messing with Druid business" -- hell, most their idle dialogue is: I'll drive these outsiders out to protect our grove, they shouldn't be here.
They're only curtailed if you kill Kahga without redeeming her, realising the fatal mistake of siding with the Shadow Druids.
I'd like it if Larian made the ones that are Vehemently against her and the Shadow Druids (Rath, the old lady, the Elf that's working with the Bird outside, 1-2 of the younger Druids) join you up top - but as it is now, I'd say about 2/3rds of the Druids actually believe Kagha is right and were looking forward to driving the Tieflings out of their midst.
How did it get to this point? Is Halsin just a terrible but idealistic leader that everyone secretly despises?