1) The metric system is the lingua franca of measurement, far more than English is a lingua franca of languages. As such, a basic understanding of metric units and usage is a necessary part of being educated and literate. In many circles it is a necessary part of being conversant. While traditional units of measure are still in widespread use in the United States and certain other places, metric is the standard. It is practically everywhere.
2) The metric system is simpler and easier to use and understand than traditional measurement systems. Because metric uses prefixes, there is effectively a single unit for all measurements of a certain kind (such as length or volume). For an example of how this would be a nice change, I have seen:
• ice cream measured in pints,
• sand in cubic yards,
• backpack capacity in cubic inches,
• milk in gallons,
• strawberries in dry quarts,
• detergent in fluid ounces,
• laundry baskets in bushels,
• refrigerators in cubic feet, and
• reservoir capacity in acre-feet.
Moreover, in many of these cases there is not just one unit that is always used. For example, I have seen news articles describe large amounts of water using acre-feet, cubic feet, and millions of gallons. Using metric units in these cases would greatly improve communication and comprehension. In metric, all of the above would be measured in multiples or submultiples of the liter. (This includes the cubic meter, since a cubic meter is a thousand liters, or one kiloliter.)
3) Metrication does not need to be overly expensive, and often it is costly to not metricate. A number of countries already prohibit the importation of products with any non-metric indications on the labels, and more countries will follow. If a company must carry two different product lines just to have metric sizes in one country and non-metric in another, that is wasteful and unnecessary.
4) It is inevitable, ultimately. The world has decided on metric. At the moment, about 95% of the population of the world lives in a country that is metric or getting close. These people represent 70-80% of the world economy, a figure which has been rising for 60 years.