Might depend on the system. Both 3.5 and PF1 have long bows with range increments of 100ft. Short bows are less. Fireball range starts at 400ft + 40ft/lvl, or a minimum of 600ft. A smart mage will start shooting while the archers are still 5+ range increments out. In most systems, mounts do tend to be ablative armor or living distractions. A spare mount back in camp doesn't help much when yours is shot/eaten/incinerated out from under you during the attack. And if the spare is with you, it is probably dead from the same thing that dismounted you.
Don't know which system or if it was an optional thing but I seem to remember a brief time when mounts could level with the characters to avoid becoming mostly useless in the mid and higher levels.
