What people who do that don't realize is that pulling late nights and weekends for a deadline more often than not just validates badly planned deadlines that were never going to work out. It's a vicious cycle: if you sacrifice your personal life to hit a deadline, then the people in charge of setting deadlines get a false idea of what "normal" is and you'll end up doing the same thing next time.
It's not that there's never a valid case for working after hours (like, say, a dedicated on call rotation), but I'm pretty firmly opposed to doing so to meet a deadline. It's unfair to yourself and your co-workers to teach product/management/whoever that work can be planned out on the assumption that your team will pull late nights to make it happen on an arbitrary schedule.