1. Lugging your adapter just isn't an option. Stop. This should be enough reason. Seriously. Don't try to justify it in hindsight.
2. Since I had to constantly borrow my adapter from my regular headphones it happened that I forgot them connected to another headphones. So I often had my headphones with me but missing the adapter. Wasn't frustrating, promise.
3. I don't know when I'm gonna need it. I made a long trip by train and thought that some nice bluetooth noise cancelling phones would be nice. Well, at my destination we took a drive and I was asked to play some music. Ooops, no adapter.
4. At work my bluetooth headphones lost battery, no worries, you can still drive them with a headphone jack. Ooops, no adapter.
5. I dance, one day when arriving to practice the people responsible for it were running late. No worries, just connect your phone! Ooops, no adapter.
6. If your going social gathering with friends to a cottage you might not think to bring your headphones. But while there some music would be kind of nice. Ooops, no adapter.
Situations like that happens all the time, and it's nothing one thinks about because it's just a given that everything works. Well, was... So now I actually did bring my adapter even in cases when I didn't need my headphones. Which was another source of (2).
The above inconveniences all assume that it works as expected when you do have your adapter. But it doesn't. You can not charge and listen to music at the same time and while I love USB-C to bits it is way too clunky to be used while in your pocket. The connector did actually start to show signs of wear from having the adapter connected while walking. Luckily I had no issues charging my device but I actually don't know if data still worked.
Think about it, that alone is also completely unacceptable.
And that's all assuming that it would be kind of okay to even have an adapter at the end of your cord, even that just isn't.
See (1).