I think it pretty much depends on your working style, and a DAW should cover every way of working. For instance, using split tracks is not a problem if you're the kind of person who gets their MIDI down and then goes onto mixing without touching it again (you can just hide the MIDI track once you're done). But let's say your working style is different. Your music creation process constantly switches between composing, performing, arranging and mixing. Some people like to get rough mixes happening as they're composing, because what they write is very much influenced by the sound of the track as it progresses. You might experiment with different synth sounds & reverbs (atmosphere). Arranging can quite easily involve elements of mixing. Should a pad be stereo or mono? If I make it mono and pan it to the left, I've made room for another part I had in mind on the right. Or just keep it stereo and stick to a simpler arrangement.
All I'm saying is that not everyone composes, tracks and then mixes in that strict order. There's a lot of to and fro, and the stereo/mono button might get used a lot in the process. Instrument tracks keep everything tidy and they're an excellent idea. I just don't see why they should be "crippled" in any way, because there is clearly enough room in the inspector to provide us with the full controls. They're separated into different tabs, while preserving screen estate in the console and track view. I work quite haphazardly and having 2 tracks per part doesn't help at all.