TL:DR - You're not.
Reviews also tell Amazon and iBooks and other booksellers, "Hey, this book is getting attention, you should promote it to other people." They help readers decide whether or not to buy a book. They can help legitimize a book (if someone is thinking of buying a book, human nature dictates they'll be more likely to buy one with 100 reviews over one with 2 or 3 reviews). In short, reviews are awesome.
With that said, I have seen a few people lately talking about reviews. This is natural when I have so many indie authors and awesome supporters on my friends list. Inevitably the talk often turns to things that reviewers shouldn't do, how not to review, or things that should be off limit to criticism. To me, this is dangerous. Why? Because in the current climate, with authors and readers/reviewers more accessible to each other than ever before, it causes tension. I know plenty of readers less likely to leave a review for fear of "doing it wrong." Here's the simple fact: Reviews are opinions. There is no "wrong" (except of course a personal attack against the author, but that's just basic human decency).
These "doing it wrong" posts often boil down to a couple of reasons people "shouldn't" review a book (or shouldn't drop a star from a review). Now, I can't speak for everyone, but if I'm on the fence about a book, I look at the bad reviews first. And if the reviews are, "This book stinks because of this personal opinion I have," and it's a personal opinion I don't share, I'll usually go ahead and grab it. (E.g. "This book stinks because the hero wears a pink tutu at one stage and I don't think the hero should ever wear a pink tutu" would probably have me rushing to buy the book. Why is the hero in a pink tutu? I want to know that story!)
So what are some of the reasons someone "shouldn't" review? I've listed some below, but it's by no means an extensive list of reasons I've heard/been told.
The argument: You shouldn't review if you don't like the genre. (Why are you even reading the genre). Variation: If you don't like the tense/POV style/etc
My counter-argument: How boring would like be if we all read the same genre constantly? If we didn't try to step out of our comfort zone once in a while, how would we find new books? New authors? New genres that we'd previously had prejudices against for whatever reason. I don't particularly enjoy spy dramas, and yet I loved Dan Brown's books (let the judging commence LOL).
If someone is given a book that their friend is raving about, let's say a paranormal romance, but the first person doesn't like paranormal romance there are a few ways it could go.
1. The new reader might read the book and love it regardless of the elements they don't like.
2. The new reader could discover a new genre they thought they didn't like but really do.
3. The new reader might read the book and think it's okay, but what might have been a 5 star read for a fan becomes a 3 star read for that reader.
4. The new reader might find the book reinforces everything they hate about the genre.
So if the new reader hates it, should they not review just because they don't like the genre? In my opinion, it's their choice. If they want to write a 1 star, scathing review spelling out all of the reasons they hate the genre and why that booked sucked out the butthole... they can and should. That sort of bad review isn't going to affect the sales of the author. Not really. I mean see above, that is my buying habit, and I know I'm not alone in that buying habit. If someone reads something outside of genre and doesn't feel comfortable reviewing it, then they shouldn't. Note: it's totally up to the reader to decide whether or not they feel like they should review.
The argument: Some authors can't afford an editor, so please don't down-rate books for bad grammar.
My counter-argument: Please do. Honestly, please, please do. I am a fussy reader. I don't want to work for my books, I like to sink into a book and just enjoy it from start to finish without having to try to sort out what the author is saying versus what the author meant to say. This is not to say typos won't sneak into the book. They will. Even traditionally published books get errors like that. I'm talking about constant incorrect word usage, constant pronoun confusion, shifting tense, head-hopping, These are all things that I want to know about. If people want to review and knock down a star for this, go for it. If they don't feel comfortable, they don't need to. If they want to contact the author direct instead, then that's a personal choice too.
Didn't finish the book:
The argument: You can't review a book you haven't finished.
My counter-argument: I get it. This is a tough one. And highly controversial. In fact, it's probably one of the most hotly debated ones. How do you know you don't like a book if you haven't read the whole thing? Maybe the author turns it all around in the last 10% of the book and that 10% is awesome and the best thing ever. But if you have to fight 90%, are you really going to enjoy the book overall? Can even the best author turn it around that much? If you don't feel comfortable rating a book you didn't read all the way through, don't. But if you want to tell everyone why you couldn't finish...well, that's fine too.
So, how should someone review?
However they want to. That's how. Even if they don't feel comfortable reviewing at all, that is their right as a book reader.
It's awesome if you review. You keep doing your thing, and do it however the hell you like. Out of courtesy to other readers it's better to avoid (or at least flag) spoilers and say why you (dis)liked the book rather than just repeating the blurb, but I'm not going to say that they are must do's. There is no "must do" when it comes to reviews. Review however you want, with whatever rating system works for you, for whatever reason you choose.
The most important thing is to have fun reading, because reading is awesome.
Tell me, what do you think? Do you usually leave reviews? Is there anything you personally don't like to leave reviews for?