Basically, it makes sure that successfully cast a spell is always a 'good' result. There are times, and spells, than when successfully cast on a low number, aren't terrible useful. Sure you lowered Mind Control to 5, but cast it on a 5, and it is probably going to be mostly worthless. It just helps ensure that wizards are the most powerful and essential pieces on the table.
If you don't like it though - feel free to play it the original way.