Two great places to farm up Charms for the mount.
1-Slags in Halls of Lightning. Decent drop rate, solid respawn rate, you can solo your way there on normal (or stealth past the trash on heroic), and if you don't kill the two fire adds at the back the respawn rate is supposedly faster.
2-Little purple elementals in Throne of the Tides in the elemental gauntlet leading to the last boss. It's possible to solo the first boss on normal, dodge the second boss, and as long as you don't kill the last two elementals at the end of the hallway, these things respawn quicky and have an excellent drop rate. Paladins can just afk with their ret aura on, and come back every 5 minutes to heal themselves.
As for why one would want to farm for the mount?
It's BoE. Wait 3 months until after the event and sell it for some million gajillion gold. I'm aiming to get 3 by the end of the next two weeks, already most of the way for the first one.