It may not have been designed for audio files, but it's pretty damn good at them anyway - the hydrogen audio chaps rate is as equivalent to AAC and vorbis at the same bitrate, as well as having excellent quality at low bitrates along with low algorithmic delay. It appears to be a "cake and eat it" codec at present.
Not quite. It's true that for the majority of western music it performs just as well as AAC and Vorbis, however there are certain classes of audio that it does poorly with, in particular polyphonic music. This is an inherent limitation (steming from the pre/post comb filter), that cannot be overcome in future encoders.
For streaming audio, this isn't a big deal as it is somewhat of a corner case and people don't hold streaming audio to flawless standards. However, for a music library, you want an audio format to encode anything you throw at it to transparent level of quality, without thinking about the technical details or limitations.
Now the problem that#s always plagued vorbis... will we see widespread hardware support for it?
Opus uses less computational resources than Vorbis, to the point where doing it in hardware is almost pointless for a smartphone (especially for streaming where the radio will be active and using more power than the encoding/decoding), and ultra-low power dedicated MP3 players are becoming scarcer. So it's less of an issue that it was in Vorbis's day.