Highlighting can be helpful so long as you know what to highlight. The important thing is to know how to pick out key sentences. And if you want to know how to do that check out our posts "The Key to Breaking Down a MCAT CARS Passage: Rhetoric" and "Identifying the Key Ideas in MCAT CARS Passages Using Rhetoric."
In any text you’re only going to have somewhere between approximately 13 and 15 key sentences containing key ideas. So if you want to highlight them, go ahead. But only them. Don’t junk your brain up with a lot of facts that really don’t go to the argument or what the author is trying to prove. So highlighting can be very effective as long as you know what to highlight.