The question arises every now and then whether one should review a novel if one cannot say something good about it and give it at least a three star review. I usually find myself in the opposite side of the fence in these discussions, urging people to use the full rating scale. Why? Let me try to explain in the following.
First, I must say that I have a long history of writing game reviews for an online magazine called Hookedgamers.com. Game reviewing is kind of public service where you try to give your honest opinion about a game so that others can trust your word and perhaps follow your advice on what games to play and what games to avoid. It is very necessary to use the full rating scale in this role simply because a game reviewer cannot just not write a review if the game is utterly bad.
Book reviewing is a little bit different, I admit. Unlike in game reviewing, I don't have to review every book I read. But rather than not review the books that I've finished, I still like to share my opinions about them. Perhaps it is the game reviewer in me, but I see it as a kind of public service as well: my opinion, if well based and reasoned, can well help someone else make a decision whether to pick a book up or not. If they find that my reasons for giving a book a bad score are not reasons that they might share, they may ignore my review and read another one to find one written by someone who shares their likes and dislikes. But if they find themselves agreeing with my reasons, they might have just avoided reading a "bad" book.
Let me share an example: I recently read a historical fiction novel set in the late 17th century. There were other reasons why I did not end up liking it, but one of them was the simple fact that the author called the protagonist's sword an épée. Whereas the word means 'sword' in French, its meaning in English is very different (modern sports duelling sword, similar to foil) and should really not be used of 17th century swords. A period swordsman would rather have used a rapier or perhaps a smallsword.
For me, such inaccuracies are annoying and immediately drop the rating that I'm willing to give to a novel. Someone else might see them as minor things and thus ignore my complaints with a shrug. Thus, providing the explanations for my rating is more important than the rating itself.
However, I also have to point out that I do not generally review a novel that I have not finished. And I leave lots of books unfinished. Often a Kindle sample is enough to show me whether I want to continue reading a book or not. Those that I do continue and actually finish are already better than "bad" books. So, in essence, a one star review from me already shows that the book was good enough to finish. So, there must have been something in it that kept me going until the end.
This goes together very well with the rating system used in Goodreads where I write most of my reviews: 1 = did not like it, 2 = it was ok, 3 = I liked it, 4 = really liked it, 5 = it was amazing. Note how a 1 star review is simply "did not like it", rather than "this was the worst book I've ever read, it was bad, bad, bad!". Simply saying "I did not like it" already indicates that someone else might - this is just my opinion.
So, these are my reasons why I prefer to use the full scale of scores when I write my reviews. Not everyone will necessarily agree with them - and if you don't, I'll gladly hear your reasons and points of views!