This is a useful new feature for teachers and students, particularly when reviewing essays. Comment-only focuses the peer reviewer. There are only three things to do: read the essay, highlight words and phrases, and leave comments. That’s it. There’s nothing to write or correct on the document itself.
For the document’s author, there’s more focus, too. When you get back a reviewed essay, all you have to do is click on the highlighted parts. This brings up the corresponding comment. After you’ve read the comment and revised your document, you click on the comment to resolve it, and it goes away. Simple.
Comment-only also preserves the integrity of the document and its author. It reduces the likelihood of plagiarism because other peers or adults cannot make modifications directly on the document.
Overall, I think this is a great idea for Google Docs. I am hoping that Google Docs will extend comment-only access to folders, so that an entire folder of documents can be shared this way at the same time. That way, a student can drag her document into a shared folder with comment-only access instead of having to remembering to share it properly every time.