I don't know how it works on WP8, but my guess is, that it's the same as on WP7. And if that's the case: I don't think it's useless. At least for me. I use it a lot with my WP7.5. Use it for music identification, searching the web, scanning QR codes and stuff and so on...
Sometimes I wish there was a possibility to activate some kind of a "app-related-search". Something like: you are using App XY, which contains some kind of a media collection (just an example). If you tap the search-button, you can search your collection, or tap it again to activate the normal "wp-search".
But that's the only thing I miss here...