[Guest]

Could you give a good reason for the need of them being three sided

Oleg's laws,don't ask me.
Ou,and by the way the 3 sides need to be alligned for smoother effect on the objects mapping.
It's not me who decided this.
To get a proper answer,why don't you ask OM via e-mail?
So,here is how the 3 sides should look like: