Originally Posted by Radiation Joe
Just playing devil's advocate here, but:
If you know what you are doing, and have a specific goal in mind, I don't have a problem mixing and matching. Spring retailers create a set-up based on what they think the average buyer will like. You may not be the average buyer.
I personally like a very stiff front end and a soft rear end (no comments on that please). When I ordered my springs from Ground-Control for the e46, I was able to set them up to my preferences.
However, if you don't know what you are doing, you can end up with a very strange handling car.
This is a great point, but as Joe mentions, it's extremely important to know how different spring rates are going to affect the handling bias of your car. Your safest bet is to go with the package deal.