I am against the error of requiring a priest to be a member of some kind of association with promises or vows ON TOP OF whatever they are obliged to do under Canon Law.
"Obliged under canon law " like what ? Like the obligation to not be on his own and to be under a diocesan bishop or a religious (or assimilated) superior ? This is just canon law, not something extra...
I do agree with a lot you said on pushing that too much and the evil side of some kind of sectarian spirit, and I also agree that priests who without fault of their own were thrown out, or had to leave their diocese/congregation/society would have valid reasons , for a higher purpose, to not follow this purely canonical prescription, but the case is different for young seminarians who start their priestly life: they were not thrown out of something, they have no excuse to not follow canon law, even when it prescribes you to be under someone...