For programming, I don't find the multiple cursor thing very useful, but for markup (especially html), it's extremely useful, particularly when working with legacy systems that don't have very powerful server-based building of HTML from templates. In these cases, HTML can be very repetitive, and grabbing a set of <li>s that you want to add a class to is as simple as highlighting the first <li>, hitting Cmd-D a few times, and then editing them all at once.