Apologies @radicalblues, I never saw this thread when it was created. Unfortunately it's difficult to see every single thread.
The issue is that star ratings were added well before we opened them up to partial stars, and the old property is still there for compatibility's sake. It should work if theme developers bind to StarRatingFloat instead of StarRating. However, when images are used in custom themes for the stars, depending on the method used to display them, partial stars may not be supported by the theme (I've seen several different methods of converting the numeric value into the visual representation of the stars, and not all of them will work with partial stars).
So it's really a custom theme issue, and depends on using the correct code for displaying the stars in the theme. To be honest, though, I haven't checked the default theme to see what it's doing, either.