I do use Marnid and Borcha quite a lot since, as it has been said, they cannot die. They are both now around level twenty five. Marnid is a skilled horse archer (scoring headshots as if he had an aimbot

) while Borcha is a horseman with a shield and a sword. They are
very helpful although they sometimes tend to get stranded in a group of footmen.. but what the hell, they cannot get slaughtered anyway

We travel alone and we don't have problems dispatching anything or anybody that comes into our way (I'm level 37 or sth). I prefer to equip them from looted equipment although I already spent some money on their weapons (to make them more effective). I like to join one of the sides of the conflict and concetrate on 'punish raiders' missions - 4500 exp. + prisoners that you can take to your group as soldiers

it's a win win situation

.. We get to kill a lot of high level units, we have lots of loot, we get huge amount of exp and a promotion usually

To level them up at the beginning I simply let them train on low level enemies.. Went into a battle.. left around five enemy soldiers and then ordered my companions to charge.. I watched conviniently from a distance with my bow at ready to step in if something had gone wrong.. They did well.. and even against worse enemies.. they at least got the share of the experience.. and once you get to finish a 'punish raiders' mission.. levels go up like mad

They are a fine distraction at the very least.. I'm a horse archer mainly.. I focus on archery and one handed weapons.