If a user doesn't like an update, he/she might say so and give it a very low rating to try to kill your sales until the issue is addressed.
Something seems wrong about this. A single user can have way too much power over sales. And there is a total lack of respect for the developer.
Perhaps Apple should allow users to go back to previous versions if they don't like an update.
One comment talks about a Monkey Island update that destroyed users' saved games. The comment remarked it didn't seem fair to rate the game down from 4.5 stars to 3 stars for something that would get fixed on the next update.
The app may have been 99 cents, but the user's saved game represents a significant investment of time (the most fundamental unit of value since all of us have only a limited supply).
The app may be only 99 cents but costing a user hours or days of investment is going to get you ranked down, and should.
The other point devs miss is that the absolute ranking of their app doesn't matter. What matters is that all apps are in the same system. If users rate your app on delete, they rate everyone else's app on delete too. If you're getting a lot of bad reviews from that, perhaps more people are deleting your app than other apps.
This is one reason free apps rank lower -- they're deleted more often, hence ranked more often, and ranked at delete time, so ranked low. But even so, free apps are ranked against other free apps, so your stars are only relative to the other free apps' stars. A star itself is meaningless. How your app does within the same system as the other apps is what matters.
TL;DR: Delight your users or at least respect their time.