This opens to market for many more developers, as there are no App Store approval processes to deal with, or any installer packages for users to stumble with. The browser is the platform, so any smartphone with a web browser is open for business. Wait, there is one "app store"--Google recently announced its own Chrome Store, which will feature applications which run in the browser, but unlike Apple, there is no approval process. Make it, market it, sell it.
While it may seem like there is no downside to browser-based mobile web apps, understand that with some apps, native applications have an advantage. Native apps can access the lower level system tools of the platform they are developed for. If your app is designed to access tools such as the iPhone's accelerometer or gyroscope, you will need to use native code to do so.