- You can promote from Disciple to Recon: Disciple of Leaves --> Ranger (They lose Potency so wait as disciple to gain exp first?)
- Allow you to cherry pick ones that spawn with Council of Esus and promote them to Recon line. (If you have all the techs and buildings, and don't manually go through Disciple of Leaves but jump straight to Ranger, they shouldn't lose Esus. The religion gives recon units extra abilities.)
Do Lightbringers start with Medic I? Because, for me, the biggest reason to upgrade Disciple to Ranger is to get March. Without Medic they will need to start with 10 exp to get it.
Also it seems to me that the advantage of upgrading from Lightbringer drops sharply after just a few... How many units with Sentry do you need?
I suppose you're trading for ...