Why? I don't understand the use case here.

Well perhaps I stretched lines when I created a page using Microdata this way but it's an example of conversion from RDFa and there it worked.
On one hand, it would also reduce the number of special cases: where present, @datetime would be always considered. But there is more.
I use Schema.org vocabulary for simple structured data on a webpage. IDK if it is too much of an indication, but I use a Schema:Webpage and a something else inside the page (usually an article with reviews) and I wanted to highlight both a Schema:Webpage.dateCreated/dateModified (hoping that search engines use this piece of data to index and refresh pages basing on recent changes, too) AND e.g. a Schema:Review.datePublished. In order to do so, with RDFa I usually pass as relevant date for the page the <ins@datetime> within which the review is shown. Then the <article> of the review constitutes a Schema:Review and the <time> element inside it conveys the relevant date for the article.

Can you give an example of where you would do this?

I'm finding it hard to understand why a page with <ins> would have a different microdata model than the same page with all the <ins> elements removed. The whole point of <ins> is to indicate that a part of the document was inserted, not to change the meaning of the document. When would you use itemprop="" on <ins>?

Please re-open if you respond to comment 3. Thanks.