Two advantages for me: It's nice that you don't break the connector if you trip over the cable or put the laptop down on a soft surface, and it's nice being able to charge while still using both USB-C ports (although I guess 3x USB-C would also solve that).
I don't really see any downside to a proprietary connector if you also have the option to charge over USB-C as well.
I don't really see any downside to a proprietary connector if you also have the option to charge over USB-C as well.