I believe that each item asked in an exam has positive backwash effect on students' learning process; on the other hand, each item you have not asked or ignored creates a negative backwash effect.
It is really hard to consider wash back as a positive factor in testing. Without research over every specific item that we want to test, it is absolutely impossible to be optimist and have this hallucination that wash back is totally for the benefit of students. I usually consider it as a negative factor because wash back has its psychological effects on students during the test, immediately after the test ,and for the long run.