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 wonder how people would feel about:
MagSafe + 2 USB C, all on the left
vs
3 USB C, 2 on one side and 1 on the other
I don't care much about MagSafe, but it is sometimes annoying to have to plug everything on the left. If given the option, I might pick the extra USB (which could also be used for data/monitor/etc. when not being used for charging, of course.
MagSafe’s great because nobody in your house will run off with your cable to charge their phone or tablet or Switch controller or whatever.
Every time that happens buy two more cables. It will stop happening.