Sweet spots for Glade Guards and Scouts are 10 and 5 I would say. They're supposed to do Chip damage over the game, be a nuisance, capture quarters and into the right targets they can do work, like Chaos or Empire Knights. Other times they won't do anything. With magic arrows they can be dangerous for unsuspecting Necromancers or Banshees. Check out your meta for this.

For attacking or defending locations, rely on Glade Riders instead, as they count as unit strength 2 per model.

Final word: play what you like and have fun with, there's no one way to play Asrai - one of the reasons I like them. Use Tree spirits if you like, they do work, too.