What, exactly, did Xeph do that would make following him seem like a good idea? Let's go over that interaction again;

Xeph: I'm going to go nark on ork and misc.!
Ork: *Tries to kill Xeph*
Xeph: *Doesn't kill vulnerable ork* Life debt nonsense! I'll kill you for self-defense! Still going to nark!

... Yep, nope, doesn't really make following the tech marine (towards the enemy) seem like a good idea. Especially when compared to being left to his own devices so he can summon up some actual allies.