It's perfectly fine because we all understand, in general, how good of a game the passer had. The closer to 158.3, the better they threw the ball. The lower, the worse they threw the ball.
It isn't the only metric you use to look at a QB. That doesn't exist. You have to take into account many different metrics. Passer rating is one, and it should be one. Part of the reason why it should be one is because everyone is used to it and immediately comprehends what a good and bad passer rating is.
I like passer rating better than QBR, because I think that inflates rushing stats, but that is another metric to consider, with all the rest. Nobody should look at only one metric and expect to understand how a player performed. They need to all be taken together.
It doesn't have any flaws. It does exactly what it purports to do. The flaw is in the interpretation. If someone looks at passer rating and expects to completely understand how a player performed just from that, they are misusing the metric.