Because the Raspberry Pi is basically an experimentation/testbed platform. It's meant to do basically anything which is why one might install Inferno, or any other OS on it.
Furthermore, it's backed by an SD card, so it's not as if any OS you choose completely monopolizes the platform. You don't need to choose just one when dealing with it.
If your Raspberry Pi is your only computer, then yeah, maybe this isn't for you, it does clearly say beta release on the page.
