You are right on with this - why do people leave switch plates on when it's just so easy to take them off? Another thing that happened to us - we had the popcorn ceiling taken off and the ceiling painted white. When the job was done and I later went to change out all the fixtures/fans, they had left every fixture in place, so there was a large patch of popcorn and unpainted ceiling left behind - it just didn't dawn on me to specify that they take those down before scraping and painting. It was kind of a mess.
I used to work at Sherwin Williams. The best thing to do is specify that you will purchase your own paint. If the painter objects strenuously, he was plaanning on making money in the ways this article outlines. You might pay a few dollars more for paint, but you will have control of the quality of what goes on your walls. Never skimp on paint quality.