If a single vendor gains control of the "market" for web browsers, as Microsoft did for a while, you are at their mercy. If there are multiple, competing web browsers and control of the APIs (e.g. HTML, CSS, JS) is decentralized, you are at the mercy of the whims of the marketplace, but no one person can wake up in the morning and decide to shut you out.
The concerns raised in this post are isomorphic to Apple's concerns about Flash. They don't mind developers using web standards to develop apps because no one vendor controls HTML, CSS, and JS. If developers write their apps in HTML, JS, and CSS, Adobe cannot flip a switch overnight and break every app on iPhone because their proprietary tools no longer emit a flavour of JS that runs on iPhone without errors.
So...
Proprietary Platforms are for Suckers.