Jon,
They dont use plastic because the seamless gutter guys have rollforming machines that make gutters in any length they want......on site. If they were plastic, they would have to be pre-made and shipped, therefore raising the cost.
The streaks on the gutters have nothing to do with what type of siding. Like was said above, they come from a reaction with the shingles. It is perfectly normal to see this on a fairly new house.
I have the perfect picture for this occasion........but I cant find it at the moment. I posted it one other time somewhere, but I dont remember where............ it is a picture that shows.....no PROVES this theory. Basically, it is a picture of one of my customers gutters where the gutter extends under an overhang, which makes a section of gutter where no water is flowing off of the roof into it..........most of the gutter is gray and disgusting, all except for that two foot section that doesnt get rained on.......it is still as white as the day it was put in.........like I said, great picture.......I hope I find it.