An advantage of shopping period over add/drop is that kids who take the summer to decide what classes to take aren’t disadvantaged. Also, a couple classes I took raised the enrollment cap and moved to bigger spaces as shopping period went on (Salovey’s Psychology and the Law notably started in a space that held about 200 people; final enrollment was about 1,500). Also also, for discussion classes where just making it bigger wasn’t feasible, it was unusual for anyone who desperately wanted to be in the class to be shut out.
That said, Yale’s extended drop period, which allows a drop with no transcript indication as late as midterm, is IMHO more useful. Two weeks isn’t always long enough to know if you’ve signed up for a heavier courseload than you can handle.