There seem to be relatively few USB/MIDI keyboard controllers with "audio interfaces"...what are the advantages of having that audio interface? If I had a USB mixer in my 'mini' studio, wouldn't it be redundant?

Just a little confused as manufacturers/resellers aren't spelling that out too much.

Well, they are good for small, low-budget setups. In particular, they are good for people who mostly use software synths, but need an audio input to record the occasional guitar or vocal or whatever. They are also good for people who use software samplers and make their own samples.

I'm currently using an Edirol PCR-1 in my micro-studio setup (most of my gear is in storage while we're building a house). It's been handy, although only having a two-octave keyboard span has been more of a limitation than I thought it would be. But it fits in my computer cart.