Compromises. The "Downside" is for the manufacturer developing many different spring and damper setups for different markets. A car is generally designed with loads of different markets in, buyers in those markets have different tastes and requirements for different roads etc, manufacturers (generally) set up their cars to be a jack of all trades. (a perfect example of this is the Corsa b, whose setup was designed to be just ok on everything, from uk back roads to bouncing across the Australian outback). It's cheaper for them to make a suspension system that is "ok/pretty good" with one development cost than it is to make 20 or 30 for different markets. Then there's the problem of replacing parts, any issues that occur in one markets springs, not knowing what others may be effected or something etc so it's easier for the manufacturer to keep tabs on it too.