Like OP, I need to underline when I read now. It keeps me focused on the book. It also forces me to re-read, to slow down and appreciate the words. This is mostly true of long/dense books; the first novel I seriously marked was Ulysses. Some books I can't mark because there would be too much underlining (The English Patient), but in that case I also don't have trouble focusing.

I can't mark a book with a perfectly clean spine. But I love finding underlining and marginalia in used books, and I generally try to buy used.

For some of the books I love, I have two copies: one marked, one clean (a loaner). The only thing that bothers me about underlining and marginalia (now that I've gotten over my knee-jerk need to keep books clean) is embarrassment if I lend books I marked on years ago, because I was an idiot.