Supporting an alsa output plugin does not always create a library dependency on alsa for the OSS plugin to work, mpg123 for example. And when the OSS plugin is configured to be the default, supporting alsa can be unnoticable by OSS users.
OSS4 has certain advantages like not locking the sound device to a single app at a time, and not creating library dependencies that could not be satisfied by either sound system. But I have cards that will only work with ALSA, others only work with OSS. And some apps like stella will not give proper sound at all with OSS, but that is unusual.
Until there is total support of apps and cards given from either sound system, there are cases where one would need to use one over the other. And even then, it is good to have a choice.