Almost every book blogger I follow has written about their views on reviewing books that they didn't actually finish (or DNFs), but I've never written abuot my own opinion. After seeing a couple more of these posts recently, I decided that maybe I should do so.
There are reviews here on my blog of books that I haven't finished, though recently I have done this less often. I haven't stopped doing so because I'm against the idea of it. I've just decided I don't want to spend my time writing reviews for those books. The entire thing would often be negative, and unless there's a good reason for it, I'm just not in the mood to go on about why I hated the book without having anything good to say about it. Still, I make the decision over whether I'll review a book on a case-by-case basis.
That being said, I almost always give any books I abandon a star rating on Goodreads, and I support people doing so. While I get why some people might consider it unfair, I don't see it that way myself. If I disliked a book so much that I couldn't finish it, then I think that's a valid experience that should be expressed through a rating or review.
Sure, there are other people who would undoubtedly love the book even if I couldn't finish it, but the same is true if I read an entire book and still gave it a negative rating.
Say a book has ten reviews that are average or positive, which makes that book's overall star rating rather good, but what you don't know is that another ten people disliked the book so much that they didn't finish it and didn't leave a review at all. I think that, in a case like that (albeit an unlikely one) that book's star rating is inaccurate, and it would be helpful if people could see that there were just as many people who disliked the book so much that they abandoned it as there were people who enjoyed it. I don't think that's unfair information to add into a rating. Ratings are, after all, to help people find books worth reading, and adding DNFs into the rating helps that.
If there were another option of Goodreads, Amazon, etc. where I could publicly mark the book as abandoned instead of giving it a rating, then I'd happily do that instead. (I know you can make your own DNF list on Goodreads, but I mean something that people would see while looking at the book information.) Maybe there would be information available on the book's page such as "this many people abandoned this book". I think that would be helpful, and I don't necessarily think it would need to be displayed as prominently as the average rating if it was there for people who wanted to see that information.
As that option doesn't currently exist, I'm going to keep rating books I abandon because I think that's information that's helpful for others and is as valid of a rating as a rating from someone who read the entire thing and gave it a negative review.
The other issue surrounding abandoned books is whether or not you add them to your "read" shelf on Goodreads (therefore letting them contribute to your numbers for the yearly reading challenge). I typically mark abandoned books as "read" if I finished more than half of the book. I think that that's fair enough and don't feel bad doing so, especially since, with the varying lengths of books, that can sometimes mean I read more of that book than another book that I actually finished.