Hi Tony,
I don't think it is a good idea to encourage respondents into thinking that they can always change their responses once they have submitted a questionnaire. You should really "educate" them and stress that clicking on the Submit button really means "I am sure that these answers are what I want to submit". After all, when they are submitting a quiz or an exam paper, there is no way to go back and make changes. I do agree that a survey-type activity such as the Questionnaire (or the Feedback) module(s) are not the same as an exam or a quiz and can be more flexible.
You can always use the "Respond many" option in Questionnaire. If you use that option and your students complain that they have to re-type all their answers when they just wanted to edit a couple of them, you can always tell them that it's their fault (which it is).
We must also consider how the teacher will use the questionnaire data. If there was an option #3 as mentioned in my answer to the OP in this discussion, and students were able to edit their answers again and again, that would seriously affect the questionnaire's statistics. How would you know which is the student's final version of their responses? Would you want the latest version to replace the earlier ones? Should all earlier versions be (automatically) deleted? Actually even using option #1 (Respond many) poses that problem. I never use that option myself and frankly I do not see much use for it in real life.
Joseph