[mcswell]

> if you can demonstrate comprehension in the absence of this structure, > I think you can make the case that it is optional and therefore may not > rely on it.

One kind of example demonstrating the importance of structure is wh-movement (the appearance of a word like 'who' or 'what' at the beginning of a sentence, when the argument it is asking about would be somewhere deeper inside the structure). For instance "Who did John say that Mary had a fight with __?" (I've represented the position of the argument with the __.) It's been known since the 60s that there are lots of constraints on wh-movement, e.g. *"Who did John say he knew the person who had a fight with __?" (vs. the non-wh-movement sentence "John said he knew the person who had a fight with Bill.")