I answered yes to the first three, because I find myself doing it for a variety of reasons at different times. The most common reason is to keep track of my place when someone calls me or walks in to my office.
The next most common reason is related to the first reason, and it's a shame that I have to do so. When you use text highlighting to track your reading (line by line), it's a sign that the site you're reading has poor typography. Usually:
Measure is too long - when lines of text are long, our eyes have a difficult time "tracking" back to the next line.
Leading is too small - Leading is the space between lines. The space between lines creates a negative void that our eyes follow back to the beginning of the next line. Not enough leading and the line is thin and hard to follow. Too much leading and your eyes get lost in a sea of space.
Even when typography isn't bad I've always had a hard time tracking back to the next line both with text on paper and on screens. My most common use is to highlight one line at a time, although I sometimes highlight to save my place (arbitrary length selection) and sometimes to emphasize a concept out of a larger mess of text (context based selection length).
Usually I highlight from near the middle of one line to near the middle of the next line. If near the end of reading line N I highlight from the middle of line N+1 to the middle of line N+2, I can use the one highlight three times to help me track back: first I track back to the first line above the highlight (N+1), second I track back to the highlighted line (N+2), third I track back to the first line under the highlight (N+3).
Similarly highlighting multiple lines at a time it's easy to subconsciously remember which line of the selection to read next (first, second, third, ...) without having to visually track back, as long as the selection isn't very long (much like how it's harder to keep track of where you are in a longer paragraph).
After a few days of use, I customized my readability (actually I use clearly now. http://evernote.com/clearly/) style to make highlighted text easier to read. After I moved to a theme with dark background and light text, the contrast between selection text and selection background was annoyingly low.
The next most common reason is related to the first reason, and it's a shame that I have to do so. When you use text highlighting to track your reading (line by line), it's a sign that the site you're reading has poor typography. Usually:
Measure is too long - when lines of text are long, our eyes have a difficult time "tracking" back to the next line.
Leading is too small - Leading is the space between lines. The space between lines creates a negative void that our eyes follow back to the beginning of the next line. Not enough leading and the line is thin and hard to follow. Too much leading and your eyes get lost in a sea of space.