I had the original wow folder.

Then decided to make six copies. (I named them wow1, wow2, wow3, wow4, wow5 and wow6)
One copy would be my one-box raid folder, with every option cranked to the max.
And five copies would be my five-boxing folders, with most options all the way down.
One of these five had maximum view distance, and that was my boxing leader toon.

A symbolic link is essentially pointing to another folder and saying use the information found here.
So you can have several wow folders, which are only 10-15mb in size, and only one original folder which is 10-15gb in size.
As far as speed goes, running from one folder or symbolically linked to one folder is the same speed.

I've personally moved away from multiple folders.
But if you're not using virtual config files, multiple folders is a good way to maintain different options for each account.
And it will remember which account was last logged in, for your B.Net if you have several accounts on one B.Net email.

As long as you break the links before a patch you're fine.