Curerently, they can't because they need to be in range, but a BLM or SMN still has their weapon drawn when casting anyway. A WHM or SCH still has their weapon drawn when they cast heals, and could remain targetting the boss to deliver ranged auto attacks while using mouseover cures. If you are targetting something while casting Bio II or Fire IV, between casts, you could swing your Staff and auto-attack from afar by shooting a little bolt. Just saying Mage weapons have good auto attack, but mages are rarely ever in range to use it, so why not give it range and let auto-attacking work that way for mage weapons like how BRD and MCH auto attacks have range.