Do you use......or abuse a hi-liter?

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I've always found hi-liters to be an extremely useful tool. For students, it has proven to be an invaluable study aid. But can hi-liters be abused to the point where their purpose is defeated? For example, I have seen text books with entire pages hi-lited. What's the point? The way I see it, the purpose of a hi-liter is, well, to highlight. To isolate pertinent information; to separate the wheat from the chaff, if you will. How many of you nonstudents still use a hi-liter? As a nonstudent writer, I use one all the time. Indeed, it is still an invaluable tool for me. How about you? In what way do you utilize a hi-liter? Do you ever abuse it? Do you have a color preference? A brand preference?