I'm on the fence on this one. Have never cheated myself but blanked once in an exam and had to leave even though I knew I should have been able to do the exam with my eyes closed.
On the whole cheating thing, know some people who were tempted, not because they didn't know their stuff but just because they panicked. It nearly destroyed them. So really each circumstance has to be taken on its own merit. Exam pressure is a dreadful thing. Exams should be a test of ability, not endurance.
We all know those who just could not be arsed and the others who panic under pressure. More often than not its those who panic who are caught. The real assholes are too slippery to get caught.