"Should you lose your spear, you need only shout." Assuming the Outsider decides to listen, but now he's made the offer. It doesn't hurt to court allies when you're in a strange land, and this man did, at least, help him first. It also doesn't necessarily hurt to show to people that he will help but will be a problem if they decide to attack him instead of letting him help.

Of course, the Outsider doesn't know about Templars yet.

The Fade Step that has replaced his teleportation, at least, should be somewhat familiar. He comes out of it just ahead of Mal with borrowed sword swinging, inhuman strength severing a demonic leg in one blow so that Jayne can pounce easier, if he'd like.