For nearly a decade now, the U.S. Marine Corps has been using diesel powered M1030M1 (Kawasaki 650cc, KLR650) motorcycles on the battlefield. The KLR650 itself came out in 1991. The marine versions are used for reconnaissance, security and courier duty, as well as getting key people (artillery forward observers) quickly around the battlefield. The U.S. military began using motorcycles again in the 1980s, having dropped them after much success in World War II. The bikes have become very popular in combat zones, and several thousand military bikes have been purchased so far.
The M1030 diesel powered bikes replaced the less powerful, gasoline powered, Kawasaki 250s. The diesel engines are not as lively as the gasoline ones, but they do supply better mileage (40 kilometers per liter, or 100 miles per gallon, and max range of 650 kilometers on a tank of fuel.) The 370 pound bike has held up well in the combat zone, and is easy to transport by aircraft or landing craft.
Noting the marines success with this bike, it has been bought by the U.S. Army and the U.S. Air Force. Several NATO countries have also bought the bike (as the M1030M1E). Four years ago, a civilian version became available, costing $19,000, and became quite popular.
Motorcycles have been used by the military for nearly a century, since the time when motorcycles first became available. Motorcycles survive because they are useful, easy to maintain and don't require a lot of fuel. The military bikes have proper mufflers, so they make less noise than larger vehicles. They are popular with recon troops and military police. In both cases, a couple of guys on bikes can keep an eye on a lot of territory, and do it cheaply and quickly. The bikes can also handle very muddy, or sandy, roads that give four wheeled vehicles trouble. In short, the motorcycles survive in the military because they make themselves very useful.