Comment 44 for bug 1542944

The co-existence issue is known. But the default for AR3012 is disabled, because it caused issues on some adapters.
And also not all Atheros Wi-Fi adapters have bluetooth chips.

It can be enabled manually as you did.

Regarding enabling it by default or making some quirks to get it enabled for some chips you can ask the kernel maintainers.
It is not too hard to do. But I am not quite sure there are still no reasons not to be done.