I use a mixer most of the time, but not because I'm missing any mids. Sometimes I have another person or two work with me, and sometimes I use my Roland E50 for small, quick jobs and on those occasions I need a mixer. I just leave it permanently in my setup.
I have the Bass boosted just a little on the Audya internal e.q., but the mixer is run flat.
I have heard of only one or two people talk about missing mids, and none of them are Bose owners as far as I know. I just don't hear anything missing.
I wouldn't be surprised if some people are using the old "V" e.q. curve that was necessary or at least desirable with the older sound gear. In that case they are boosting the lows and highs, and in effect lowering the volume of the mids.
At any rate, the e.q. is easily set and saved on the Audya. If you want more mids, just turn 'em up!
To answer your basic question, the mixer is certainly not necessary for me at all 90 percent of the time, when I play alone.
I could also use the second mic input and line input for a duo as well.
But I like to have the mixer along JUST IN CASE the Audya crashes. I could use my laptop for midi files and run it and the mics through the mixer and make it through the job if needed.
Audya display is 4&1/2 inches by 3&1/2 inches.
DonM