John:
This is an excellent question.
The site for Rock Creek in Montana has some power lines in view at the bottom of the course ... they are high up on a hill, and cross the valley below the 17th green. They're hidden from some angles by rock outcroppings, but you can see them from probably half the holes if you try, and you can certainly see them when you play down the 14th and 15th and 16th holes if you want to look in that direction.
For a while, we rejected that site altogether because of the power line view, and looked elsewhere on the property for the golf course (we had thousands of acres to choose from), but we didn't find another spot with nearly as many good natural holes. After some deliberation we went back to the main valley and worked on the routing with the goal of avoiding any holes which played directly toward the offending towers.
Does it spoil the experience? Not one bit, in my view ... after a few trips around the place, they just fade into the background and you don't care at all that they are out there. Will it hold back the course's ranking? Possibly, because there are some people who just look for some reason to knock every contender, and this gives them an easy reason to dislike the course. [The same guys would rule out Royal St. George's as a great course because of the nuclear power plant down the road, except that some would make an exception because the power plant came after the course, and because they wouldn't want to look stupid.]
Certainly, there are degrees of exposure to such visual blights. I was amazed to find that Firestone had a power line running through it, because you NEVER saw it on TV. Of my own courses, Riverfront and Quail Crossing have to deal with power towers -- the ones at Quail Crossing are much bigger, and closer to play because the land plan had to keep them away from the houses.