Well, ALSA Being Unnecessary Is Arguable
Well, it's arguable that ALSA was unnecessary. There are still some people who argue that the latest OSS would only need a little work plus a lot of open drivers added to be a better choice than ALSA. I don't know that I'd call ALSA's OSS compatibility exactly a 'layer,' and it doesn't seem to cause too many extra problems above and beyond the issues ALSA itself has.
However, a sound server is not really competition for ALSA or OSS. Windows has a sound server (and not a terribly good one) along with its sound driver API framework. ESD and PulseAudio are generally not used together (though it is possible to do so). It's basically like any other thing in the open source world. At most levels there are competing projects available to perform the same function. That doesn't mean that there are generally more layers of software running at the same time on a Linux system than on a closed system.
Generally, on a Linux system that is used in a similar fashion to a Windows system you will find equivalent layers of functionality. Windows has a sound driver API framework, and Linux has ALSA or OSS, not both at the same time (though ALSA offers an OSS compatibility module that you can load to catch OSS calls, mostly for legacy software). Then Windows has the Windows Sound Server, and on Linux you have the option of running PulseAudio (common these days) or ESD (usually called EsounD now) or another sound server. So we really have two basic layers to the sound system on either operating system.
Of course it's possible with either Linux or Windows to dispense with the sound server. It's also possible to add a low latency sound server like JACK with or without the basic sound server on either system. Most people will only do things like these if they have a particular need though.
Edit: I'd like to add that ALSA was invented because of some basic missing functionality in OSS, which was very much controlled by 4Front Technologies. Since that time 4Front has adjusted the license to OSS to make it easier to develop for, though they still offer quite a number of extra drivers in the closed source version of the software.
