Thanks for the reply Knut!
"At least for the vocals they need the preamps. I doubt "amateur home recorders" have preamps of the Babyface quality.
The keyboard is usually a stereo device and could not play/record with guitar/vocals on only two channels at the same time."
-Well I'm an amateur home recorder and I have two preamps with as good or better quality then the Babyface (allot of people have bought into the 500 series models).
-I never meant that the keyboard would be played with guitar (I'd like to see that), I meant one or the other with vocals. But you do have a point with keyboards usually being stereo.
"There is the RME Babyface. But an universal interface for mobile and studio recordings - like the Babyface - is useless without preamps. There is no "better" quality and there are no unecessary extras. The great success of the Babyface shows that it provides exactly the feature mix that 2-channel users need and want - no less, but a little bit more."
-I would also guess that most amateur/hobbyist home studio recorders are not doing mobile recordings.
"An interface with 2 in + 4 out of this quality would be not really cheaper without MIDI or SPDIF/ADAT I/O. But it would be a lot more inflexible and this means not RME-like."
-I'm all for the inclusion of midi with either adat or aes (does anyone really use spdif anymore? I've never used it) for flexibility. But USB would be the main interface.
-What most don't need are the extra convertors and preamps...and they do cost more money if you want quality.
Also, I'm not a fan of breakout cables. I would also guess that if people had a choice, they would prefer not to have them.