There are other apps that allow you to stream your local music if that's what you want to do (AllCast, BubbleUPnP, etc).
But I've always wondered about this argument - Why wouldn't you put your local music library on Google music? It's free and means you've got access to your library everywhere. I could understand before they had matching that uploading 100+GB of music is probably not desirable, but it now even big libraries are fully uploaded in a day or so.
I don't want to have to upload a huge library of music just to stream it to my local network, where I already have the music on my hard drive.