Your concern is reasonable. Creating too many routing rules in Yii (or any other frameworks) will degrade your application performance. This is really a trade-off between performance and functionality.
The benefit of using URL rules in Yii is that it offers you two-way URL management: URL parsing and URL creation. Apache rewriting is only for URL parsing, so are most other frameworks.
As a result, Yii developers can always create URLs in the same form: createUrl($route, $params). And the generated URLs can be changed easily to different SEO-friendly forms by changing the URL rules. They no longer need to hardcode URLs in their code.