Well, rating someone 4/5 is only a problem if everyone else rates 5/5 for merely acceptable service. So perhaps we should evangelise for lower ratings, rather than squeeze everyone into 5/5?
It's in this vein that I rate 3* as a base (get me from A-B without crashing or similar incident) and go up/down from there based on the overall experience.
There's people in this thread stating they've given drivers who've provided bad or unacceptable service 5*, just because. It is, as you state, this behaviour which is the root of the problem in these systems.
5/5 - No problems worth mentioning.
4/5 - Minor problems.
3/5 - Major problems.
2/5 - Unused.
1/5 - Unused.