Back at Gamescom2017, Ion Hazzikostas, @WatcherDev was answering questions on twitter. One of which was, “Isn’t now the best chance to add animations using the staves for caster classes (at least optionally)? That would be so great.” -@AlexCC94
The answer from Ion was, “Spells in Warcraft have historically been conjured, not channeled through weapons.”
I’m going to show my age and point out that weapons were used for caster animation in Warcraft 3 and they looked cool.
Uhhh… ok, maybe not that troll one in the middle… but the others look good! Most of them are even just weapon swings with a particle effect. It would seem like something like that might be easy to accomplish with cosmetic glyphs or transmog options.
Weapons could be used to denote a difference in what is happening, such as casting a spell vs channeling a spell. You could make any number of arguments for when weapons would and would not be best utilized in casting animations, but I think all it boils down to casters wanting to see their weapons used in any kind of way other than a stat stick that never moves from their back/hip.
No matter how epic and awesome looking your weapon is, it is diminished if you literally never use it.