Just to clarify some FUD: things like USB audio work perfectly well with OSS. Things like mixing and resampling work actually better in FreeBSD OSS than with Linux ALSA, since the latter can't even handle multiple audio streams transparently unless hardware supports it, or unless one uses some userland cludge, not to mention e.g. transparent resampling or parametric equalizer. Supporting UPnP, which Lennart has mentioned, is not the job for kernel.
In short - BSDs don't use ALSA, because it adds lots of bloat without much added functionality. The only reason to use it would be backwards compatibility with Linux, and that actually could happen some day, unless everyone migrates to PulseAudio and stops using ALSA directly.