Steel lasts longer in a working environment. Poly doesn't like running over sharp edges, at all. Given the chance steel will cut a groove in sharp edges. I don't think poly is stronger when you remove all the Safety Factors and just look at Ultimate Breaking Strength. It wasn't when I looked at this a year ago or so.
I don't see poly on tow trucks. Cost driven? Life span driven? Who knows. When I start to see poly on a lot of tow trucks I'll re-think this.
Lest you think I don't like poly, the reason I looked at it was that I was looking to see if I could get away with running a smaller OD poly as a way to get more length on an unmodified winch drum. Two friends & I did some testing of 1/4" Amsteel (Not the higher strength Amsteel Blue) figuring that was the worst case scenario. It was pretty impressive stuff. Learned that no matter what you can NOT tie a knot in it. Knots create the same heat under load that a sharp edge does and the poly line just won't take that.