In 2007 DST changed and Mexico did not follow suit, so the short PST/EST/CST timezones were not specific enough anymore. Here is an excerpt from Wikipedia (http://en.wikipedia.org/wiki/Pacific_Time_Zone):
Effective in the US in 2007 as a result of the Energy Policy Act of 2005, the local time changes from PST to PDT at 02:00 LST to 03:00 LDT on the second Sunday in March and the time returns at 02:00 LDT to 01:00 LST on the first Sunday in November. Canada would also acknowledge this change. In Mexico, beginning in 2010, the portion of the country in this time zone uses the extended dates, as do some other parts. The vast majority of Mexico, however, still uses the old dates.
If timezones aren't good enough to display the time, what good are they? If Mexico is calling PST/PDT a different time than everyone else, shouldn't they be using new values instead?
The timezone names most countries use are not unique or entirely stable as time goes by. E.g., America and Australia disagree about what "Eastern Standard Time" means. That's why the Olson/Eggert database names zones for major cities, those are most stable and well known.
