The main problem with this has been that the DAI cable often is (and should be!) attenuated. Thus, the earbud would be super quiet if you just attached it in this manner.
It just occurred to me that the middle pin could be connected as unattenuated. Earbuds would then connect to the ground and center pins, and ignore the attenuated pin.
To illustrate, it is this
but with the V_dd pins connected to the respective left and right connections on the stereo jack.
As far as I can recall, I've never seen this pin actually connected to anything - it's just there for robustness and/or ease of insertion of DAI connector into a hearing aid's boot, I think? Has anyone ever seen a situation where this would be a bad idea? Obviously this is only really relevant if you're making a custom DAI connector (does anyone besides me do this?...).