Gameplay Meta aside, personally, I just hate the look & feel of a mighty Sorcerer having to use a mundane Crossbow… it just doesn’t fit. At all.
On the other hand, a staff that shoots a projectile with a suitable animation and hold style, that fits perfectly and moreover mechanically it would be 100% identical to a crossbow!
That latter point is the main argument why it just makes little to none sense to force casters to use mundane weapons like that… mechanically and balancing/rule-wise it makes 0% difference wether it’s a crossbow shooting metal bolts, or a staff shooting some kind of magical projectile, so long as underneath they are identical in terms of damage, CD and GCD, etc… or even different but balanced properly, for example with the staff’s magical projectiles having a small armor penetration bonus, or ToHit for magical creatures, whuteva, but in turn a lower damage value ([…]) than a crossbow’s bolt.
That way the look & feel would be 100% perfect (especially since those of you who for some reason prefer a crossbow-wielding-caster could still do that), the investment in terms of manhours should be relatively small as there’d only be a couple extra animations which also could largely be repurposed (magic missile projectile recolor and maybe resize, for example), but it would give those players who really care about these things a much improved immersion.