OpenAL supports per stream volume. The service is basically needed if an application wants to 'audio groups' all using the same sound card across processes, and be able to switch it to some different card with 1 command. Since I don't think the sessions are persistent, this requires a audio service no matter what audio api is used. :)
Speaking about Cross-Process
Does wasapi offer any ways to manipulate(e.g. volume control) or access another processes streams? With a sound server this might be possible, don't know if it makes sense though. I guess openal cannot do this because its just a per-process library.