I've got the app but it is not what WP needs. Play to needs to be built into the OS just like airplay. An app should be able to send music to a "play to"-compatible receiver for example Xbox 360. Everything that is not as good as airplay is not acceptable. The Nokia app is not even able to play all music in my collection.
Do you realize that baking an app in the system makes it impossible to update it often and when needed? The file formats that play to can reproduce is limited from the target device, where files would be reproduced