If a candidate uses an electronic spell checker no one will be in a position to tell them what kind of spelling errors they make. Some people only misspell difficult words in which case it could be good to advise the candidate not to use unnecessarily complicated vocabulary in the examination room. When you write a TOEFL essay I think you should do so under examination conditions. Only in this way will it be possible for instructors and/or examiners to make suggestions for improvement.
You make it sound as if a TOEFL candidate would be able to focus on one thing only at a time. I think it’s possible for someone to use a spell checker when writing business letters but refrain from using one when revising for the TOEFL test. And this is precisely what I suggest TOEFL candidates do. Don’t use a spell checker when you write TOEFL essays as you will not be using one in the examination room. Do be all means use a spell checker when you create documents for other purposes.
I am aware that you, Torsten, are an excellent English language trainer and so I do not seek to offend you personally. I just got the impression that you are intentionally misinterpreting what I am trying to say and I don’t find that highly professional.
All the best