It's a cool concept, but if you had to wait for a mob pop in a quest that only spawns during the night, that'd get old quick or vice versa. There's also other things that factor in that are game related to in-game time.

Some NPC's may only appear at night as well or during the day so it could be hard to complete something if you're going off of real time. And remember these servers may be region based, but we're not forced to play only on a server specific to our region so if you set your time to CST in US and you're playing with someone in England, it just wouldn't work out too well.