Compression and Damping rates. If your fronts have different rates than your rears, the handling can be severely compromised. Just think of it this way... If you are taking an off camber corner at speed and run over a pot hole, and lets say your fronts are slower compressing than your rears, the front will be more affected by that sudden jolt, perhaps causing snap understeer, your car starts pushing to the outside of the corner, and HELLOOOO GUARDRAIL!
Just an extreme example, (no, NOT from experience!) but based on my Autocross experience, an unbalanced car will NOT behave as neutrally as it should, ESPECIALLY a Bimmer, which have always been noted for exceptional handling!
Hope this helps!