Every time these types of things pop up, there's always people extolling the superiority of the metric system.

Long story short: a lot of units have their pros and cons, and while metric is the international standard and "simple" to remember, it isn't necessarily superior in every day use in a lot of industries and areas.

First off, that map is inaccurate - a lot of countries use a mix of metric and customary units. Canada and the UK come to mind immediately. Hell, in the US, you can buy soda in 2 liter and 1 liter bottles, but milk comes in gallons.

Second, it a lot of things depend on the context for which they are used.

Cooking, for instance, is a pain in the ass figuring out cups vs. tablespoons vs. pints and so on. On the other hand, it is easier to scale recipes up and down using those units because they're conducive to fractions. 3/4 of a cup is easier to measure out than .177 liters, for instance.

Aviation is another area, for instance, which is officially metric. However, all but a handful of nations (China, North Korea and Russia in particular) use feet and nautical miles.

The reason being? A nautical miles, before being officially pegged to 1852 meters in the 1950s (which isn't exactly a easy # to remember), approximates 1 arc second of latitude, useful for navigation purposes. In addition, 1 nautical mile is very close to 6000 feet.

Why is 6000 feet better than 1852 meters? Well, in aviation, timing matters a lot in navigation, planning descent profiles to save gas, etc. - and time is measured in 60 seconds a minute. See how the math is easier to do with a nice round number to divide into?

Of course, that corresponds with altitude (measured in feet), which also allows for easier math. ATC also separates aircraft in altitude using 1000 feet, which makes for easier math than 300's of meters.

As you can see, many things are relative, but metric is far from the more practical application, nor is it universally superior by any means