Isn't there a difference between writing a review and merely rating a book? The star ratings are just a way for people to say whether they liked a book or not, not a rating based on an objective study of the merits of the work in question.
Once I've read about a 3rd into a book and I can't stand it, then it becomes fair game to say that I did not like the book IMO. And that's exactly the equivalent of the one star rating according to Goodreads: 'did not like it'.
Fair enough. I've never been on Goodreads, and there is a bit of a distinction between a review and a rating, though in my mind not a very big one. If you're talking about some kind of multi-paragraph/page in-depth review, compared to a single click 1-star rating, yeah there is a difference. But to me the difference of 1-click and a basic (maybe a sentence or two) 'review' of why you did/didn't like a book are very nearly equivalent.
Less information does not make it less important. When you search for a book to read, I'd imagine you generate some big, maybe genre-wide, list and sort by rating. The ratings you see could be 100 well thought out reviews, or 100 people who never even read the title of the book who click a rating for fun. Is there a difference? Of course there is, but the outcome for the person scrolling through the list is the same.
But thats more my general opinion of online rating systems and not specifically about this situation. I've been whined at for not liking plenty of books/series. Its not like my personal opinions about a book are some kind of objective, be-all-end-all 'this is what the book is' statement.
Give it whatever rating you think it deserves. I'm sure people have given better/worse ratings for fewer reasons (didn't like the cover art, stupid title, didn't read book but don't like author, etc. etc.)