When can you say that technology is useful in education?

Any time something is of measurable benefit with few or no adverse effects it can be called "useful". Technology has been useful in education for quite a long time now however. So called "super computers" were being used in collegiate education in the late 1960s, giving way to the evolution of spacecraft, computers, home electronics and literally hundreds of the biggest and most profitable companies in existence today.

Technology is the future, and keeping children educated on the subject is their only hope for financial and social success, generally speaking.

A shorter answer to the question is this: You can say technology is useful in education NOW.