There are plenty of reasons to have computers in schools.
For one thing, word processing is generally much quicker and more efficient than writing things by hand. For another, there are still some children who don't have computers at home in poorer areas. Not knowing how to use computers as the 21st century goes on is going to become like being illiterate in the 20th century; school might be the only place where those children learn how to use computers. then there's disabled children who have trouble with things like writing; computers can be incredibly helpful in these situations. There's plenty of educational software around as well; this has the advantage of being interactive (unlike textbooks) and being able to engage each child individually at the same time (unlike teachers); I get the feeling that there's going to be lot more decent educational software coming out as the role of computers in schools increases.
Needless to say, there are disadvantages to relying too much on computers in schools, especially if the teachers have trouble with technology, but it certainly isn't the case that there's no reason to have them in schools.
Also, I'm not sure whether "in Japan" was supposed to add any authority to the claim that computers shouldn't be in schools. Japanese schools may get amazing test results, but I don't think we should be copying them.