That's a good point. However, if we1 can put in heuristics to the PM code to decide when code tags are required (and are not actually present), and we decide these heuristics are accurate (i.e., zero false-positives, few false-negatives), and we decide that this would be beneficial to the community at large, then we could make it mandatory: no "submit" button until all warnings are dealt with.
I would propose that there also be a way around these heuristics. That is, I like the computer being smart. But not too smart. For example, perhaps if you submit a preview without having changed anything, then you get the submit button on the next preview. Or, perhaps, there is a checkbox saying, "ignore warnings" which, if unset (default) would make the submit button act as the preview button.
And then, any time warnings are bypassed, perhaps we could automatically consider the node for those warnings? :-)
1 I'm using a very, very loose definition of "we" here. As in, "we" == "gods".