Apparently they make up ~7% of internet users. So my guess is, yes.

It really depends on your target audience who will be using your application.

If you can block them (ie: the user needs to login), you don't need to wrory.

It's also important to note that you don't just need browser support. You also need support in every web appliance that the request passes through.

It's really common for web sockets not to work through corporate content filters and old load balancers.

> Does anyone really care about IE 9 any more, though?

Not Microsoft. It's unsupported as of 2 months ago.