Another factor to think of, in Mmmystery on the Friendship express it was demonstrated that Swords could be held by hooves. Unless, of course that was simply Pinkie's insane imagination. Considering all the other things that we've seen as hoof-held, I don't think it's too far of a stretch for a sword. Also this:
The frog is similar in shape and consistency. Some biological adaptation using that method is the most plausible explanation for hoof-grip.