Routing units

[Poulp']

If you let them go, they will fall back, regroup and merge on a friendly territory.
To make sure they won't come back, you have to hunt them down.
AFAIK, it gives no particular penalties, you'll just eventually gain the "bloodthirsty" trait.

true, routers bring less experience than fighting enemies,
but when it's time to chase routers, you no longer are here to proove your worth, are you ?