In the early game, it doesn't make sense to use trait tomes on your companion but by the late game, there will be situations where a single trait point won't help the main character out, but would be nifty for a companion to have, same with scrolls that increase skill points.
It really depends on your build, your companion's build and how that lone trait point might impact overall game play. Keep in mind the every increasing cost of trait points to raise a trait one level.