The Gregorian calendar with which we are all familiar with today uses the extra day in a leap year; other calendars such as the Hindu calendar are based on the lunar system and introduce the 'leap month' in order to synchronize with the solar year. The extent to which calendars struggle to preserve the lunar and solar concept also reflect, to a large extent, what was important to the cultures creating them: harvest cycles, lunar tidal forces, nomadic timekeeping, or a mix of those and more.
I'd assume the Mayan 'baktun' to be an indication of an astrological calendar, as their 'end of year' predictions follows the astrological cycle present in almost all cultures at some point or another: The baktun is when one constallation 'sets' and the next 'rises' as the dominant constallation in the night sky. The heralding of a new era, confirmed by all other astrological calendars as well – we'll be leaving the era of the Fish, and entering the era of the Ram.
It may be fun to have a 'baktun-in-review' on December 21, highlighting mankind's most significant events in the last few hundred years. If anything else, it should help us realize how many of our problems are history repeating itself, and how insignificant we truely are in the larger timescape.
Oldest Mayan calendar found, and it goes way beyond Dec. 12, 2012 (+video)
A Mayan calendar was found deep in the Guatemalan rainforest. But this ancient Mayan calender refutes claims that the world will end Dec. 21, 2012