Taco, coils on coilovers aren't progressive, as you said but leafs are somewhat progressive. Correct?

Yes leaf springs have a progressive rate, but there are progressive coil springs too. What Im trying to say in the above statement is that springs are crap for soaking up bumps.

When you apply X amount of weight they compress a certain amount no matter what.

While a shock doesnt. It does not dampen X for a certain weight, it dampens depending on hard hard you hit a bump. How hard you hit a bump depends on what speed you were going. Yes we can say the same for springs but only to a point. Sorry Im having a hard time explainning this. But they just aren't the same. Springs get softer the harder you hit them while shocks get harder the harder you hit them. Thats their job, to absorb shock. This is why its pointless to use springs to dampen the vehicle over tough terrian.

OEM's use high spring rates and soft shocks because its much easier to get a good on road ride with much less R&D. Once you start going down the road of soft springs and quality tuned shocks you increase the cost of the vehicle development. On the other hand, a stiffer spring costs just the same as a soft one and cheap crappy shocks are a dime a dozen. This is also why stock sprung vehicles typically ride horrible offroad.

Leaf springs have to be stiffer otherwise they will compress to much at ride height, wear quickly and most notibly have tons of axle wrap. If you intend to run leaf springs, the absolute best shock to buy is a bypass. This is because of the progressive spring design of leaves. With coils or coilovers it isnt needed as much because of the linear rate. I am in no way saying bypasses aren't needed with coilovers. I think for racing they are.