[millstone]

In this scenario, what's the point of having more than one OS?

[int_19h]

Same as with POSIX - different implementations of the same standardized interface that target different hardware, and have different quality of implementation (possibly optimized for different use cases).

[Jach]

Differences of opinion in what/how to run code outside of whatever browser people decide the sandbox boundaries are.

[Yoric]

For end-users, I'm not convinced that there is one.

Well, besides keeping competition/innovation alive.