I have thought about this question myself. I have a Mackie 24/8 and I am wondering whether I should dump it for
a Mackie Control. For me, the main reason is becuase I have a MOTU 2408mkII and a 24 i/o which, in 44.1 khz gives me 32 analogue ins and outs. Since I only have a 24 track board, I have not utilized the extra 8 from
the 2408 mkII.
Anyway, in considering the question, some of the problems associated with just a midi control are monitoring and auxilliary sends and returns. Becuase you do not have the auxs in the mixer, you will need to use up some of your trakcs to do this. Furthermore, if the midi control does not have a monitor secion, you are going to need to add one.
In the end, the 24/8 is, for me, a monitor control that helps me to sum up all of my keys and modules and whatever else I have recorded (a barking dog last night). So I have kept the mixer.
I think the question about summing in analogue or digital has to do with sound quality and headroom. Summing all of those tracks in analogue, I guess, would be more foregiving whereas, summing in digital might prove difficult to avoid level problems and nasty digital distortion. But, if you get used to it, the digital summing with the type of preamps you mentioned would probably sound best and cleanest.
I am no expert to be sure but, for me, if I were in that position, I would probably want to take a stab at the midi control without the mixer. For one thing, you will get a much better sound from the preamps mentioned and there are seperate monitor sections being made by some companies to fill the gap. That could do the trick. As for the aux sends and returns, it should not be too hard to accomplish this through use of patch bays and/or internal bussing and routing.