I use mine
to read the road ahead. On a decent country road that I have never been on, with hills and blind summits, it's useful to know where the corners are, and how severe they are. That over the brow of a hill is a junction, or a sudden curve to the right or left. Yes, road signs can tell you a lot, but they won't show how sharp a bend is, or how close a hidden junction may be.
They are no more a distraction than a quick glance at your speedo. Also, as others have said, very useful navigating around a strange city. Always have a map, if only for route planning and programming in waypoints, but also in case your SatNav dies.
I never use mine if I already know the road, but you will always see the same car drivers on the same route every day religiously running with their SatNav switched on. Some people can't live without a gadget I suppose.
If you can afford a bike specific unit, like the Garmin Zumo aor the TomTom Rider, you can't "fiddle" with them on the move, so the temptaion to do so is removed. This is not the case for the car units that some riders use on their bikes.