This is what Todd said in a recent interview -> here's the link http://www.oxm.co.uk/26155/bethesdas-todd-howard-on-elder-scrolls-5-skyrim/
Carelessly drop an axe in a forest during an afternoon's bear-slaying, for instance, and a merchant chum might retrieve it for you later. Behead him as punishment for running his filthy, lucre-loving fingers all over your prized weapon, and he won't be able to enlist you as a caravan escort - but his unsuspecting widow might. You get the idea.
^so he just confirmed traveling merchants with and hiring mercenaries, right? I have seen many people talk about how they want this in these forums and I'm glad it's in the game.
I hope, however, many merchants have they own bodyguard(s). I'm gonna be the one ambushing the caravan and I want a challenge.