Tyler Cowen doesn't. I do. Here's why:

1. I'm a compulsive note-taker, and margin notes are one way of wrestling with ideas in real time. This is especially true when I'm reviewing a book or when I don't have a notepad or computer handy.

2. It adds value when I give books to libraries or lend them to people. Used and marked-up copies of older books brings me into conversation (albeit faintly) not only with the author, but with a previous reader. Reading th rough Murray Rothbard's personal copy of Douglass North's Structure and Change in Economic History, for example, made an impression on me when I was at the Mises Institute in 2002 and 2003.

3. Assuming the books aren't destroyed, my marginal notes will bring me into conversation with future readers. Just as when I read marked-up copies of old books I can see what someone, somewhere thought was important, I can be that someone, somewhere to future readers.