I think your view on what PA is is a bit too simple. It's actually a little bit more than just some code that mixes audio for you. It allows you to switch streams on-the-fly between devices, does upmixing/downmixing (stereo to surround and so on), allows you to combine multiple sound cards into one, allows you to have seperate volumes for all streams you play, and so on, and so on, and so on.
The most interesting feature in my personal opinion however is the new 'glitch-free' core as described on my blog a while back: http://0pointer.de/blog/projects/pulse-glitch-free.html -- this is of course not directly visible to the user, but nonetheless a major feature of PA that you really cannot do without on a modern operating system.
Also, please note that Takashi Iwai maintains PA in OpenSUSE. Takashi is one of the two lead figures of ALSA. I think this little fact should make clear to you that even the ALSA people themselves believe in a need for a userspace sound system that goes beyond what ALSA provides.
The misconception that PA is little more than a replacement for dmix is popular, but nonetheless a .. misconception.
If you have trouble with PA's CPU load then try a different resampler. ALSA's internal resampler is very very primitive and sounds terrible. It uses little CPU, but is one of the weaker points in ALSA. Claiming that PA is a regression over ALSA in this area is probably simply caused by setting different priorities.