Dont think you can take the jaguar warrior charm if you mount your charecter.The spell only works on models on foot.
Things you can do with a flying chareter, can move him to the unit that needs him the most.
Can charge warmachines, small archer groups, skirmishers, fast cavalry, flyers, chariots and other anoying things on turn 1.
Can work as a character assasin, charge that block of a unit and kill the mage hiding in it for example.
Basicly it gives you a strong hitter that can be where ever he needs to be.
Havent used him myself yet, but that what I would use him for. Also a slann with lore of beast casting bears anger on him before he flyes away could be an intresting suprise to most things in game, a 28" charge with 7 strenght 7 attacks, could kill a lot of things ^^