2. Need to keep track of mount, one more thing to forget or lose, and possibly have to replace. Cost of having one for each of home, office, and travel bag, if that's your typical solution for reliable access to small easily misplaced accessories.
3. Need to futz around setting up mount and phone every time you want to web cam, assuming you use your phone for other things in between, and don't plan on having the mount attached to your MacBook all the time.
3. Can't use phone while web camming with mounted phone.
4. Can't use phone to cam on days you can't find your phone, or other corner cases: left it somewhere, phone battery is low (ok you can add sum extra futz for a charging cord), ...
5. Non-ergonomic and possibly physically unreliable for moving or tight situations, like when in a vehicle or a cramped plane seat.
6. Looks/seems silly. Unprofessional. Cluttery. (Worth putting in here twice!)