[Moppie]

Usually the boxes will out last the engines, and the clutches can easily do several hundred thousand km's if they are used properly.

There are also huge advantages in having more than 3 gears, and a direct driveline also means less power loss to the wheels.
All the medium duty 4cyl Fuso's, Hino's and Isuzu's sold here have a 5 speed gear box. It gives you a very low speed crawler gear for 1st, that you only really use when the truck is fully loaded, and then 4 good gears for regular driving. You do all your starts in 2nd, acceleration and really steep hill climbing in 3rd, low speed driving in 4th, and high speed driving in 5th.
The slightly bigger 6cyl models usually have a 6sp, which just gives you an extra over driven gear to make high speed driving more efficient and quieter.