Style lock!

[Braden]

Mages still swap from club+shield to staff pretty frequently.

And instruments probably won't help, as Harp/Horn have different animations they'll probably swap, and yeah, Terp/Eminent/Daur/Gjaller is pretty much the swap of choice these days.

You can already lock instruments and bells. In fact, you can lock an empty slot and sing with Gjallarhorn or dance with Dunna.

Shield blocking is probably the only reason they won't let you fully lock outside of combat, and I'm guessing the "solution" is to add more similarly functioning gear across different types (like Mafic Cudgel + Genbu's Shield = Earth Staff).