Let me explain;
MIDI instruments/pads/synthesizers are absolutely imperative to audio production. it is not a feature that a producer or musician can do without, it's not something that can be overlooked. A quick Google search on "Adobe Audition MIDI Instrument Support" reveals posts across forums whether it be Reddit, Adobe, or some other random production group forum that instruct users how to rig up a number of different work arounds to use MIDI Instruments in Audition. You'll also find a number of people who have switched to another DAW as a byproduct.
Excluding the functionality is nonsensical, the reasons that I've read primarily on this forum are as follows:
1) "Audition is not nor does it claim to be a music 'production' or 'creation' software, it's instead an 'audio editor'" - Okay? Except, no. Whether intended or not, Adobe sets the expectation of Audition being a full functioning DAW. The product isn't named "Mastering" or "Audio SFx", it's called "Audition", which provides a mental visual of Audio Production software. Also, the product supports Audio Interfaces, has recording functionality, has send/side chain functionality, it has the functionality that is found in virtually every DAW excluding one of the most important things; Midi.
2) "Audition is going down the path of integration with Premier"
And? Is that to suggest that Movies/Video media has no need of sound effects/music? I have a Library of over 4,000 cinematic themed MIDI Instruments/Pads/Synths that I would love to use to add sound effects to my videos; Integration with Premier furthers the need for MIDI, not reduces.
3) A lot of people don't want it; Or it's niche
Reference Google search above; Thousands of videos wouldn't exist if it were niche. Not to mention, there so many Adobe users of Lightroom/Photoshop that play an instrument and would love to record but don't know where to start in the mainstream world of products like Abelton, FL Studio, etc. but would likely be willing to try working with a product line they're familiar with. All in all, the people who don't want it lose nothing by having the functionality addad. The people who want it would gain everything.
I would use Audition hands down if MIDI were available. The interface, the integration with other Adobe products like Premier, etc. make it a perfect tool; Excluding it's lack of MIDI. So for now, it's back to Abelton/cancelling my Adobe subscription.