If your audio chipset doesn't have a hardware mixer, and about of half of laptop chipsets don't, then you can't play more than one stream at a time. If you have an array microphone, the only way it'll work at all is with Pulseaudio. If you want Bluetooth audio, only Pulseaudio bothers to support it.
Pulseaudio uses ALSA anyway (and on DFBSD it uses OSS instead); it's not a replacement, it is the only unix sound server other than Apple's(and maybe OpenBSD's) which actually works. ALSA is an audio device driver ABI; it can only expose what your chipset supports in hardware.
The problem of Pulseaudio spinning, at least when it used to happen on Linux, is due to issues in the individual audio device drivers. The reason this is a big problem is that Pulseaudio needs to run at a low `nice` value in the scheduler, or you will get buffer underruns (which sound like pops and clicks). Unfortunately this means that if Pulseaudio goes into a hot loop, it will consume all of the available CPU resources until the loop breaks. This problem can be mitigated by running pulseaudio with a control group to limit CPU time (as is done with systemd on many Linux distros). Unfortunately, DragonFlyBSD doesn't have the same thing configured, assuming their kernel supports it, so you get 100% CPU spinning.
Furthermore, DragonFlyBSD doesn't have ALSA; they have OSS.
