Companies offer unlimited vacation because it is an effective way of reducing the amount of vacation that employees take. A fixed number of vacation days should be seen as minimum, not (only) as a maximum.<p>This works because the formal level is eliminated ("I have X vacation days per year, but I have only taken X-Y days, so I will take Y more days before the end of the year."). Instead, employees need to defend each vacation against their manager.<p>Needless to say that insisting on vacation will not look great when it comes to discussion about promotions, salary etc., especially in comparison to a co-worker who has taken significantly less vacation. Hence, "unlimited vacation" becomes a race towards zero vacation.