Just for the reading comprehension to highlight key points. Yellow highlighting is much more visually appealing than pencil underlining. Also, I don't think a little bleeding through would be very troublesome, although you have a very good point. Most of my friends who have taken the test had no idea that they could have used a highlighter. Several said they wished they would have known because they would have made use of it.
And to solve the problem of bleeding through, couldn't you just bring an older, more used, highlighter? Usually they don't bleed through when they get older. Or even bring one of those mechanical multi colored ones? That way a visual distinction could be made (if it bleeds through, use a different color the next passage) I found the rules on LSAC and it said "no mechanical PENCILS", it didn't say no mechanical highlighters.
Has anybody here used one on their LSAT?