1 hated it/was offended/didn't finish
2 disliked it/likely didn't finish
3 finished but it was a struggle
4 finished/enjoyed/had some issues
5 finished fast/really enjoyed/may or may not have still had issues.
If I look back at the 30 to 50 books I read in a year, it's really difficult to get any sort of ranking out of them. I've only got two choices for books I liked (4 or 5) and two choices for books I disliked (1 or 2). Having half stars would help my ratings relative to one another.