I use my browser to read, watch, and listen, as do many others. I have never heard of people creating music with Chrome. Support for MIDI should be the last thing browser vendors should be concerned with right now.
You probably hadn't heard of people watching videos in a browser until someone made a compelling app. Same goes for so many other things that run in browsers.
The question you haven't answered is why would I want to use a browser for that? Why do you suggest that native apps are unfit for creative purposes? Browsers are somewhat unique in that it's a runtime that runs untrusted code from the internet, and I simply don't want such a security-critical runtime acquiring more capabilities without second thought. Especially for things like access to HW. Remember the fiasco with the Battery Status APIs? Remember how it's main usage in the wild was to enable user tracking?
