I'm not strongly opposed to this but I do think it adds undesirable overhead to the user experience. I do feel pretty strongly that if we go this direction, the order should be flipped. That is, the user identifier should always be requested on open no matter what. Then the answer to this question can also be logged and associated with the user who made the decision.
I still think that always tracking changes from non-blank values (including defaults) and never tracking changes from blank values (including values that were set, cleared, then set again), is easy to explain and sufficient but I certainly could be convinced otherwise.
I commented on the doc but it may be easier to respond to different parts of my reasoning here.
Perhaps it would be slightly nicer to have a "pristine state" concept as @ggalmazor alluded to. That way a change from non-pristine blank to non-blank would get a reason for change. But I really do think this situation is rare. The added complexity of keeping a pristine flag for each field seems too high for the benefit it brings.
For non-pristine blank to non-blank to happen, the user would have to blank out a value, navigate away from the field (e.g. by tabbing in Enketo or by swiping in Collect) and then come back to the field to write a new value. This is different from selecting a field with a value in it and changing its value without navigating away (this would be tracked).
The paper equivalent would be crossing out a previously-entered value, doing other things, and then entering a value again. Generally I think that would be given a single explanation. For those who have used such protocols on paper, what do you think?
There is long-press to clear in Collect and the possibility of clearing certain question types by clicking a trash icon in Enketo. In those cases, I think it's reasonable to expect that the reason for change when the value is blanked would be an explanation for why the value is being changed to a new one. I think it would be strange for the user to be prompted again when setting the value.
For those of you who think this is common, could you say more about when it happens?