Agreed, but most teams will keep a better 53rd player on the active roster rather than 'waste' the spot with a QB that is extremely unlikely to play. The 3rd QB rule doesn't apply to PS players and teams can only use the SEA rule to promote a PS player sparingly. I don't expect the Seahawks to have a #3 QB on the active roster or use the SEA promotion to add a 3rd QB unless/until either Smith or Lock is injured.
If Purdy isn't 100% it makes sense for them to have a 3rd QB. If he is 100% the 49ers might cut Allen and sign him to the PS. Allen is only being paid vet minimum but as a vested veteran his contract will become guaranteed if he is on the active roster for Week 1. It would have made more sense to have Lance on the roster as the #3 QB for Week 1. They could then trade him and have a non-guaranteed Allen as the #3 on the PS. I don't like the trade and the timing makes even less sense.
I think the 9ers are too snakebit, after all, this is the rule named for their situation. They don’t have to prioritze Brandon getting scout team reps or coaching him up. They wanna set it and forget it so they have that third butt sitting there.
Now if and when injuries strike—as They do under Shan—they Should have a qb4 to permanently elevate to take advantage of the emergency qb role, or to swap out if Darnold or Allen need relief or are benched. Ian Book, Trace McSorley, David Blough, Kellen Mond all got released and are safe stashaways